Written By: Jeremy Finison, William Shade & Mike Toth
With all the finishing or topcoat options for wood projects, I’ve always been very confused and even intimidated to learn them or tackle the right coating for the right project. So, being a formulation chemist and hobbyist woodworker, I had to set out and learn this stuff / sort it all out. To help me better understand what I’ve read and seen in videos I decided to put it in a bullet point format to help me quickly visualize the differences, benefits, and tradeoffs of each type of topcoat/finish. I’m sure there are many great books on this topic but I figured I would share what I’ve learned recently in hopes this can be a “quick reference” and learning tool for you as well.
This is just from my recent and brief research into the finishing realm that I’ve never known before. I reviewed Youtube folks I’ve watched make great projects / tutorials as well as pulled opinions from forums such as Lumberjocks & NCWoodworker.net. Also note that the inputs from my contacts at BYK - Wood Coatings Division merely reflect their individual opinions. There are also epoxy finishes that I’ve not covered here / do not know much about and have not researched them as they are much more expensive and out of my desired price range. There are also categories for wood fillers/pore fillers, stains, gel stains, pastes/waxes, dyes and tints that I’ve not gone into here as this was just meant for topcoating options. There are also many different techniques to apply each of these coatings which I have not gone into.
Acknowledgements & Contributions
Jeremy Finison (jfynyson) researched and pulled most of the initial content together for this article after various online searches and YouTube videos. The written content was then filtered through wood coatings industry experts such as William Shade BYK – Wood Coatings Technical Service Chemist and Mike Toth of BYK - End Use Manager - Wood Coatings NAFTA Region. Many thanks to the guys from BYK for their willingness to take time out of their busy schedules to answer questions, educate and help edit this content for accuracy.
Here is a good video explaining oil-based finish differences:
Varnishes (a general or umbrella term / category)
A varnish can be defined in several ways:
Any film forming finish
A cross-linking topcoat or cross-linking pigmented topcoat
A thin clear coat
All varnishes include a resin (3 types below), an oil (typically linseed or tung) and a solvent typically mineral spirits or naphtha to lower the viscosity or “thin out” and a “wipe-on” uses even more solvent. Industrial varnishes can be Alkyd, nitrocellulose modified, urea-melamine resin technologies.
Some will lump water-based into this category resin/polymer and solvent but I’ve separated it out for this discussion
Urethane resins – “Polyurethanes” are typically cheaper finishes like Minwax®; yellows more than alkyd resins and does not leave quite as good clarity of a finish but is lowest cost and still very durable for the hobbyist. Yes, these “urethanes” are not industrial 2 pack urethane systems. Probably much cheaper and use a urethane polymer with no crosslinking therefore durability is far less.
Alkyd resins - General Finishes Arm-R-Seal™ and other higher end finishing looks; less yellowing than other varnishes. However, some alkyds can heavily yellow depending on the oil it is refined from. Industrial conversion varnish type resins applied to kitchen cabinets for example are very important to have good QUV (sunlight, weather/moisture resistance) and resistance to yellowing.
Phenolic resins – spar type urethanes (mostly phenolic resins but blended with urethanes too) that are more elastic so good for exterior where could get wet (think boat parts finish but not the epoxy hull finishes); yellows more than other resins though but have more UV protection and are very chemical resistant.
Depending on the ratio of resin to oil you get into more complicated types such as long or medium oil varnishes; long oils having more oil to resin ratio and are more elastic but softer.
Benefits & Trade-Offs:
Very durable as compared to just oil, oil-varnish blends, shellac, and lacquers
Typically “cures” in 2-6hrs and is ready for next coat which is must faster than oil-varnish blends; typically recommended to sand / scuff between coats to build up the film otherwise coats may not properly adhere to one another. Industrial Conversion Varnish (CV) systems are often oven dried to help speed cure time. Catalysts used have narrow pot life windows.
Conversion varnishes are described in further detail the Q&A section
Easy for wiping on or brushing on depending on thickness & mineral spirits is typically used to adjust to your liking. Modified drying oil types, not an industrial coating. More robust, easier to use, but quality/durability is not great.
Must ensure any wipe on rags are left out to dry / cure before trashing otherwise risk spontaneous combustion. Linseed oil for example will oxidize and get hot. Most coating manufacturers require production personnel to wet rags after each shift and store in a grounded metal rag bin.
Messier clean up; need to use mineral spirits or paint thinner
Takes well to diluting with mineral spirits to thinly build up coats deeper from inside the wood (helps bring out the wood’s beauty) but if undiluted then harder to penetrate deep into the wood unlike the oils
All varnishes will yellow over time unlike water-based finishes
Bad odor during application and for 1+ months even after cured (depending on environment) unlike water-based
Tung oil, boiled linseed oil, mineral oil, etc…
Benefits & Trade-Offs:
Gives a great look but is not very protecting as compared to other finishes. Could also in some instances provide a wetter look.
Easiest to apply, simply wipe on and let it soak in
Must ensure any wipe-on rags are left out to dry / cure before trashing otherwise risk spontaneous combustion. Linseed oil especially.
Odor can be an issue for a while
Requires periodic reapplications to keep it looking good
Danish Oil such as those from Watco® or Tried & True™ and other products labeled as “Oil Finish” consists of mostly an oil like tung or boiled linseed oil and some varnish to help it to “cure”.
Contains much more oil than would a typical varnish and these are typically tinted as well.
Benefits & Trade-Offs:
Provides the great oil look but gives more protection as the varnish in it cures. Can also help to provide a very nice haptic feel (soft to the touch).
Takes ~24+ hrs depending on conditions to “cure”
Great for easy wiping on
Must ensure any wipe-on rags are left out to dry / cure before trashing otherwise risk spontaneous combustion
Odor can be an issue for a while; even a couple of weeks depending on environment
Requires periodic reapplications to keep it looking good but not near as frequently as just oil
Easy to make a DIY blend and hard to go wrong by simply mixing 1 part varnish + 1 part oil + 1 part mineral spirits (or thin with mineral spirits to your liking)
See these links for excellent videos for understanding other finishing types such as shellac, lacquer, and water based:
Category of its own; natural-based resin material that comes from the secretion of the lac insect onto trees in India & Thailand; it’s flaked or beaded and can be purchased in different tints and is easily dissolved & thinned in denatured alcohol
Known as a universal finish (further detail below)
Great option for a “seal coat” or sometimes called a “wash coat” to trap colorants (or even odors for restoration work) or preserve a desired look of the wood prior to topcoating; this helps mitigate blotching to
Benefits & Trade-Offs:
Shellac is the only known universal finish that one can apply and once cured will adhere to any other finish/topcoat type; example you cannot apply a water-based finish over an oil based or lacquer but with shellac in between coats all scenarios are possible
Sold as the waxed (great for using as a standalone finish and will adhere to itself) or sold as the “dewaxed” version (must be dewaxed if you want a worry-free seal count or used in between other finish/topcoat types
Excellent “seal coat” or “wash coat” or “sanding sealer” used to seal the wood pores for blotch mitigation for wood species such as pine, cherry, birch, etc…and mitigating grain raise if using a water-based topcoat; most sanding sealers are shellac or have it in them
It’s food grade, is in a lot of our food, used mostly for pill coatings, great for kid’s toys / kid’s furniture finish
Makes for a very beautiful finish by itself as you build the film; easily wiped on or brushed on
Dries very fast, maybe not quite as fast as lacquer I believe but faster than water-based poly and far faster than varnish
Not quite as durable a finish as a water-based poly, varnish or oil varnish blend as it’s more brittle like lacquer, maybe more brittle than lacquer but if damaged easy to repair & recoat as needed
Easily buffed out for a mirror shine finish (called “French Polishing”)
Easy to make your own cut (dilution); most folks make a 2 lb cut which simply means 2 lbs of the shellac flakes per one gallon of denatured alcohol
Mixed opinions of shellac’s shelf life once its already been mixed as it begins to degrade once in solution but Zinsser’s Bulls Eye® shellac (waxed & dewaxed types sold at Lowe’s Home Depot, Klingspors, Woodcraft, etc…) claims they now have a 3 yr shelf life; also keep in a cool dry dark place; Charles Neil branded products are highly recommended but unsure product life
·Not truly a “polyurethane” as that name is “Marketing 101” to confuse the public since we know the term “polyurethane” so well; they get by with this apparently by having just a very slight touch of urethane resin in it (not enough to do anything); these are all acrylic based emulsions
Benefits & Trade-Offs:
Very durable once cured and while maybe not quite as durable as an oil-based varnish it’s very close & likely good enough where one couldn’t tell the difference with the latest technology / formulations available; older formulas left a bluish hue but now mostly crystal clear
Does not yellow over time; this is one of the best things I like about them (I personally do not like the yellowing of varnishes); some products may use UV blockers as well
Maybe not quite as natural looking on darker woods like walnut as some have noted a very slight haze vs an oil based varnish and that it didn't seem to make the grain “pop” like oil-based (note that I’m unsure if this was using a higher end water-based finish though)
A preconditioning step of dewaxed shellac helps mitigate the lack of grain pop by sealing the pours rather than the topcoat
Always a milky white emulsion and is typically only dilutable by about 10% (can only add about 1 part water to 9 parts finish); unlike oil varnishes than are more easily diluted much farther even down to almost no varnish if desired
Dries very fast (good) so one can quickly apply many coats to finish a project and mitigate as many dust nibs settling from the shop onto the finish but that also means it can be very tricky to apply (bad) if you are finishing big projects
You want to minimize overlap and it’s recommended (as I found to be true) to pour the product into a pan and dip a paint pad then wipe on to help more rapidly apply the coat and mitigate overlap as it levels out into the previous lane before setting up
One still wants to sand between coats for the best look but too many coats of water-based products apparently can make the piece look more like plastic (I feel the same way though with oil-based coatings)
Lots of poor reviews from products from companies like Minwax (cheaper and found at Walmart, Lowe’s, Home Depot, etc… as Minwax Polycrylic™); this may be from older formulations though
Lots of excellent results from higher end products (more expensive) like General Finishes and Deft™, and even Varathanes™ (all of these are available at most wood working places & online and I see Varathanes at Home Depot); I’m a believer in water-based now
I used the product from Deft as it had built in UV protection on the huge slab white pine desk; used 2 coats of shellac to mitigate pine blotching & grain raise due to water based top coat and a single coat of the Deft product looked almost like a showroom satin
Low odor and not too foul in my opinion (many waterborne coatings contain ammonia); wood piece loses its finish odor very quickly unlike a varnish thus making it an excellent choice for indoor projects
A coating consisting of a resin (sometimes a shellac), cellulose ester (nitrocellulose mainly), or both, dissolved in a volatile solvent blend typically just called lacquer thinner (includes different blends of toluene or xylene, naphtha or mineral spirits, esters, glycol ethers, ketones, and acetates)
Primarily used in professional or assembly line type company’s finishes due to its ability to easily be sprayed on thus providing a ton of coverage in a hurry and its rapid dry time as well as great crystal-clear appearance
Aerosol cans are readily available in Walmart, Lowe’s, Home Depot, typical woodworking stores, etc…and are great for smaller projects; there are now even some “brush-on” products but these typically are hard to work with due to dry time and therefore are not very favored
For pre-catalyzed vs catalyzed see the Q&A section
Benefits & Trade-Offs:
Very rapid drying, pretty durable, very beautiful luster finish
Highly flammable and the odor is very “offensive” and very harmful to breath must have excellent ventilation / air filtration (if indoors)
Very easy to spray on and get great results on large projects (with the right equipment)
Easy to repair if damaged as each layer added dissolves into the previous lay via the heavy solvent package in the formula
Forum subject matter experts to add information here
Benefits & Trade-Offs:
Forum subject matter experts to add information here
Forum subject matter experts to add information here
Benefits & Trade-Offs:
Forum subject matter experts to add information here
Questions & Answers
This section adds a little more detail behind some common questions I had as well as some more in-depth questions into the science behind the various coatings. All of these answers come from William Shade and Mike Toth of BYK – Wood Coatings Division (Feb 2020).
Can I share this info with my local woodworkers forum ?
Yes, but please note our response(s) merely reflect our individual opinions.
Can I reference you and our discussion ?
Yes. Please also reference William Shade, BYK Wood Coatings Technical Service Chemist. He assisted in providing his perspective and technical expertise in wood coatings based on his own experience as a wood coatings formulator at industry leading wood coating manufacturers; and currently, as BYK’s technical service chemist for Wood Coatings End Use. In addition to our lunch meeting, I’ve provided some additional market context to your questions.
Varnishes / Polyurethanes
How do you define a “varnish” ? Any film forming coating ?
Mike Toth: In the industrial wood coatings market I think of a Varnish as a crosslinking topcoat or cross-linking pigmented topcoat. These acid cure systems often contain urea-melamine type resins. This is a standard technology for the domestic Kitchen Cabinet market. Acid cure coatings are normally spray applied. An organization known as KCMA sets durability standards that many manufacturers and coating suppliers conform too.
William Shade: Varnish simply means a thin clear coat. Sometimes wood varnishes will be modified with nitrocellulose which acts as an agent to give body and thickness to the coating, but it also adds outstanding clarity and visual properties to the finish.
Alkyd: Alkyds in wood coatings are polyesters and are usually made from plant oils. Some common oils used in wood coatings come from coconut, soybean, flax (linseed), and castor beans. The oils are then processed and modified to turn them into a polyester, or what is commonly known as an Alkyd. Some Alkyds are drying oils, which means the resin will crosslink from the oxygen in the air. Alkyds are dissolved in strong organic solvents such as toluene, and MEK. This helps to make it much easier to rework and fix a damaged or imperfect coating. Each layer of coating will partially dissolve and re-wet into the previous, but I still recommend sanding between coats.
2K (2 pack or 2 part) Polyurethanes coatings are cross-linked, in that a strong chemical reaction occurs with the addition of an isocyanate, which acts as the molecular connector of the polymer chains. The chemical reaction often forms a thermosetting film; which is more resistant to heat, chemical, and mechanical stress than an alkyd lacquer which usually forms a thermoplastic film.
Thermoset polymer chains have been cross-linked to form strong covalent bonds (intramolecular forces). For example, the coating may be delivered in a solvent like MEK, but MEK will not dissolve the cured coating.
Thermoplastic polymer chains typically are not interlocked by a molecular bond but are held together by weak intermolecular forces. The dry coating can partially liquefy with enough heat, or solvent. This allows flow and leveling imperfections to be buffed out by the heat generated in friction. Generally, this is what makes them more workable for repairs.
Acid cure technology combines the concepts of the strong and durable polyurethane with the workability and aesthetics of a traditional alkyd lacquer. There may be a mixture of thermosetting and thermoplastic polymers. Acid cure coatings are formulated to balance the coatings so the craftsman has the best of both worlds.”
What is a conversion varnish mean (what is the conversion) ?
Mike Toth: I think of a conversion varnish (CV) as converting a “part B” (catalyst) like a PTSA acid (p-Toluenesulfonic acid) into “part A” just before spray application in order to form a protective coating. When I think of CV, I immediately think of durability from resin crosslinking and bonding to wood substrates. Durability and aesthetics are both very important characteristics CV coatings must have. CV’s typically have a short pot-life associated with their use, so a coating must be applied in a specified operational window (hours). CV coatings typically offer performance features such as very good durability, surface protection and clarity. CV’s are typically applied on all common household wood substrates like: maple, oak, cherry, alder and hickory. Solvent-borne acid catalyzed conversion varnish is often used in the stock, semi-custom to full-custom wood kitchen cabinet market. Often, very high-end cabinetry is coated with CV technology. Additionally, it is very common to have pigmented white conversion varnish systems as well. In my opinion, I would consider acid catalyzed conversion varnish systems a differentiating step upwards in durability from traditional nitrocellulose lacquer technologies.
What urethanes are used and where do they come from (other than simply oil)?
William Shade: When I hear of urethanes I immediately think of isocyanates and 2k systems. Essentially urethanes are oils that are rich in hydroxyl functional groups (like an alcohol). These groups are the reactive sites for the isocyanate to form polymer chains in the coatings. The types of resins and isocyanates used will yield certain polymer chains and crosslink densities that will give the coating the desired flexibility & hardness, as well as, chemical and physical resistances.
There are 1k (1 pack or 1 part) waterborne and solvent-borne coatings that are polyurethanes, often these resins have urethane components in their chains. Then the polymers will form together, and may or may not crosslink.
I will typically use a Polyurethane coating in areas where I need the highest resistance to mechanical wear and tear. Such as a floor, or the top of a desk, or a chair. Their wear resistance has no comparison.
Mike Toth: 2 pack PU (2PKPU) is often considered to be a very high performing coating technology. It will typically offer higher solids from the urethane resins. Not to be confused with lower durability 1 part (pack) urethanes and other “urethane” products that might be commercially available. For example, 2PKPU is very popular for use in office furniture (i.e. tables) that are high build, and this technology is common in Europe for wood furniture type coatings. In industrial environments, the downside to 2PKPU is from the use of component B hardeners that contain isocyanates (NCO). When mixing 2PKPU for industrial finishing, the safety aspects for contact and inhalation exposure must always be considered to ensure safety for the spray operator. In today’s modern coating market, several large coating manufacturers market isocyanate-free industrial urethane coatings for use in automotive, wood and metal markets.
What’s different about wood coating urethanes vs glue urethanes?
William Shade: Both usually are made with a polyol and/or an isocyanate, however glues are typically formulated to have stronger and more flexible bonds than a coating urethane. The glues may also need to have better penetration into the wood grain to achieve optimal performance. Moreover, glues do not have to be formulated to have the aesthetic properties of a coating.
Alkyd vs phenolic resins ?
William Shade: Phenols are synthetic resins derived from petroleum distillates.
Are the flatteners silicate-based ?
William Shade: Crystalline silica is not popular at coatings manufacturers because of risk of silicosis. Amorphous silica is often used instead, it is still an irritant to your lungs, but it does not cause silicosis, which is comparable to diseases caused by asbestos.
In the old days the rubbed effect was simply polishing a coating with a certain grit paste to give a micro texture to the film that will refract light. Today, micronized particles are used to scatter the light at the surface of the coating. Not all flatteners are silica base, many are micronized organic polymers some of which come from natural products like carnauba wax.
Mike Toth: Typically yes, can be for example a fumed silica. Or treated and untreated type silicas. In contrast, some micronized wax polymers like CERAFLOUR offer good matting capabilities and surface protection features for scratch, mar and slip. Some grades of micronized waxes like CERAFLOUR-1000 are based on bio-renewable sources and offer improved clarity over silica based matting agents.
Moreover, natural and synthetic organoclays used as rheology agents in coatings can offer some degree of matting, but most likely will have a reduction in clarity. An example of a commercial synthetic clay would be: GARAMITE which is an additive for use in solvent-borne or solvent-free coating systems; and LAPONITE, for use in aqueous coating systems. These clays help provide rheological benefits to the coating promoting pseudo-plastic flow behaviors while also contributing to a matting effect. Silica offers superior matting efficiency in a coating, but the tradeoff is it can be susceptible to surface “mar” and may tend to scratch easier than a micronized wax product.
What specific silicate(s) are used ?
Often the silica is precipitated out into an intensely heated flame, and in the fumes a light ‘porous’ particles with extremely large amounts of surface area is formed. The degree of which affects the matting efficiency they have on the coating.
Mike Toth: Examples can be: fumed Silicas. Untreated and treated variations.
What is the yellowing/discoloring mechanism?
William Shade: Some resins are more likely to yellow over time, such as nitrocellulose, alkyds (degree depends on the makeup of the oils, soya based alkyds have been the worst in my experience) and aromatic isocyanates. Often times this is in relation to covalent double bonds being excited into a higher energy state by light photons, and reacting with oxygen in the air. Typically, polymers that are rich in double bonds, or aromatic structures are the most likely to yellow because of how the valence shells are excited by the light, there may be oxidation in this process as well.
Mike Toth: Resin and film’s exposure to harmful UV light acting as an accelerator
What is used for UV-blocking ?
William Shade: UV-blockers are not used often, but I believe there are some aluminum metal complexes that can be used to block UV light. We use absorbers, and light stabilizers.
Hindered light amines stabilizers (HALS). UV absorbers are used to absorb the light. i.e. Benzophenone type compounds.
Roughly what % is solvent vs oil vs resin vs other additives ?
William Shade: Oil/resin 30-60% Solvent 60-30% Other Additives 5-10%
What’s being used in water-based formulations ?
William Shade: Resins for water based formulations are synthetic polymers, often derived from petroleum, but may also come from recycled plastic or a bio renewable source. The polymers are not soluble in water, so they can be emulsified in water, or treated with surfactants and dispersed with very high shear rates to create a dispersion of micronized resin particles in the water. Upon drying these particles will coalesce to form a film.
Acrylic, and polyurethane are most common, or modified acrylic with constituents like vinyl or styrene.
Mike Toth: Over the past couple of decades conventional waterborne coatings, and more recently, waterborne UV coating technologies, are gaining popularity for industrial wood finishing. Waterborne UV is often applied by means of a flatline reciprocating spray system. This type of finishing is popular for the cabinet and office furniture markets which provide a high-quality finish with the benefit of very quick cure-to package- to ship-to shelf advantage. However, the initial equipment investments for UV coatings can be a deterrent for smaller wood finishing shops. UV coatings use acrylate resins and are formulated with light sensitive photo-initiators to bring about a full film cure when exposed to UV lamps. Mercury vapor lamps are typically used for clear topcoats to enable a good surface cure. With the current trends to many shades of white cabinetry offerings, white pigmented UV coatings are very popular, but they require the additional use of Gallium lamps to achieve a “through” cure for pigmented UV coatings. Mercury lamps are the standard for surface curing of clear UV topcoats. An alternate technology involves LED UV finishing. LED UV technologies are gaining popularity in Europe, but I would say is still very much in its infancy here for industrial wood coatings in North America.
Are these emulsions ? If so, what are the emulsifying agents ?
William Shade: Most products are emulsions, I cannot say what the emulsifiers are but one end of the emulsifier will be hydrophilic, and the other end will be hydrophobic.
Higher quality waterborne resin are dispersions. The resin particles are treated with a surfactant and dispersed in water under high shear.
How many professional shops have switched to water-based finishing?
Mike Toth: Quite a few especially smaller furniture & cabinetry shops use water-based finishes for: ease of use, to avoid or comply with local environmental regulations, and to provide or market “greener” coatings technologies. Less solvent odor during production during finishing or unboxing of a finished wood product is a benefit. Waterborne wood coatings have grown in popularity over the last decade and have much improved appearance today than waterborne coatings of the past. Spray applied water-based stains (modern no wipe and spray stains) are popular in the industrial wood cabinet markets. Likewise, in the industrial hardwood flooring markets, waterborne roll-coat applied stains are popular for use under 100% UV coatings.
Assume using acylates…if so, how do these differ molecularly what polyacylate thickeners, water conditioners, fake finger nails, etc…?
William Shade: I think of acrylates as chemicals with acrylates on the functional group. Often times regular tap water is used to make waterborne coatings, and it may be conditioned with ammonia to achieve a higher pH. However, some higher performing waterborne resins can be destabilized by the salts and minerals in our tap water and thus they must be made with water that has been filtered through a reverse osmosis filter.
There are many types of thickeners that are used, the simplest thickener is purified clays. The clays will swell up and create a lattice structure, and the fluid is in a matrix. The matrix simply collapses when enough force (shear) is applied to the coating.
In general, there are many different additives that go into water, most of them synthetic chemicals that are modified with hydrophobic and hydrophilic groups so that they can have dynamic performance and functions in water.
What is the curing mechanism here?
William Shade: The mechanism is coalescence of the resin particles as the water evaporates. Think of what would happen if you mixed a vinaigrette in a blender and then poured the emulsion onto a plate. As the water dries, the oil droplets will form back together to make a uniform oil film. Water-based resins may also have self-crosslinking mechanisms built into the particles so that the polymer chains form a strong covalent bond. They are a complex balance of hydrophobic and hydrophilic properties to stay suspended in solution as well as create a durable coating.
Moreover, there are 2k component extensions that are sometimes used to extend the performance of these resins. Typically, these are highly reactive chemicals that increase the crosslink density of the polymer chain.
How is it that they cure crystal clear after being milky white?
William Shade: The milky white are the emulsions.
The clear more translucent systems, such as a PUD are dispersed in water.
See salad dressing example above.
How do these dry so fast ? It’s not just water leaving is it?
William Shade: The crosslinking mechanism continues even after the water has left, often times solvents like glycol ethers help in this. There is a temperature that the resins need to reach for the coalescence to take place, essentially there is some energy state that the resin particles need to have to overcome the force that are keeping the particles separate.
MFFT definition and concept
What are some of the incompatible chemistries after cured?
Most waterborne coatings are likely broadly compatible after being cured.
Pre-catalyzed vs Catalyzed
William Shade: Both have the same chemical mechanism that crosslinks with the addition of an acid and an alcohol is released as the byproduct
Catalyzed or “Post-catalyzed” use a strong acid, and the reaction is generally much quicker, which means that the film properties will develop sooner
Pre-catalyzed systems are catalyzed with a weaker acid, and the reaction is much slower, it may not even need a catalyst as the reaction will take place in the drying phase. For instance, alcohol is a byproduct of the reaction and thus its presence in the liquid paint keeps the reaction from occurring because the presence of alcohol is preventing the reaction kinetics from moving forward. However, when the coating dries the alcohol evaporates from the film and the chemical reaction is able to move forward.
Mike Toth: Pre-catalyzed lacquers are just that, weakly catalyzed during the manufacturing process. They have a typical expected shelf life of perhaps 6 months (but can be less or more depending on chemistry and brand). Catalyzed coatings, like Conversion Varnishes are catalyzed just before application and have a pot life typically measured in hours. Some CV’s can be de-catalyzed and reused for application at a later production date if properly de-catalyzed. Pre-catalyzed lacquers are very popular for wood finishing on furniture, doors, baseboard moldings etc... They tend to be user friendly and offer a good price to performance balance. Many commercially available brands of quality pre-cat lacquers exist.
What are the actual catalysts used?
William Shade: Weak organic acids cut in an alcohol such as IPA. P-Toluene Sulfonic Acid is the stronger acid that is typically used in post catalyzed systems
Mike Toth: PTSA is a very common catalyst used for CV systems
Why is a conversion varnish not typically lumped into varnish terms ? (a resin, an oil & solvent) ? Is this marketing, history or just hobbyist ignorance (like me) ?
William Shade: I have heard differing descriptions about what makes a conversion varnish different.
Some regarding the balance of the urea and melamine and alkyd to get a certain early film hardness
The maximum amount of nitrocellulose that can be in the formula.
My opinion is that a conversion varnish is a varnish with higher solids 40-60%, and that develops the film properties faster. I think they are usually post-catalyzed and likely forced dried in an oven to maximize this.
I will say that wood coatings is a fairly exact science, but there is some art involved to how we describe the application and the properties of our coatings, some of the verbiage has been muddled overtime.
Mike Toth: Urea melamine containing resins are very common for CV in the industrial wood coatings market. However, a by-product of this resin chemistry can be HCHO (formaldehyde). However, there are low and formaldehyde-free CV coating systems currently available on the market. One thing to note about a CV is they can often be more difficult to repair, unlike a traditional nitrocellulose lacquer that tends to “melt” into subsequent coats. I would say the relatively lower price points of nitrocellulose lacquers and the “repairability” (ease of repair) make them very attractive coatings for both the professional wood finisher or hobbyist.
How are their also water-based versions ? Lacquers are nitrocellulose based and that’s not water soluble right?
William Shade: nitrocellulose is not used in waterborne coatings.
A waterborne lacquer is formulated to match the performance and aesthetics of a traditional nitrocellulose lacquer
Some aspects that would make a good ‘waterborne lacquer’:
It should have good flow and leveling
Should have good sanding properties
Easily spray applied
Most important is that it should not kill the clarity or adulterate the tannins (tannic acids) in the wood. Older waterborne technology was developed with chemistry that were too anionic, and thus very basic. These properties would discolor the tannins in wood, especially when applied on cherry making it look gray and dead. Moreover, waterborne coatings used to have a look and feel that made the wood like cheap plastic.
How is a water-based conversion varnish different?
William Shade: Typically, you cannot get waterborne coatings in the higher solids range like a standard conversion varnish
What are some of the incompatible chemistries after cured?
William Shade: I think in general waterborne will only be a problem if not fully dried when recoating, but some solvents may cause issues and rewet the coat, and this may make the best finish. I don’t think the compatibility can be generalized here.
Mike Toth: Examples of some common industry terms for performance failures in wood coatings, typically after cure are: adhesion failure to substrate, inter-coat adhesion failure, air-entrapment, “bubbling”, “pin-holing”, “cratering”; and coatings that “sag” or “run” when spray applied. Rheology additives play a critical role in helping storage stability, anti-settling properties and flow behavior (viscosity) of a coating, be it waterborne or other technology. Surface additives can help improve substrate wetting, levelling and provide improved surface features such as: slip, scratch and anti-blocking; and in some instances, can help improve the orientation of matting agents.
How is shellac dewaxed ?
How is it / what’s the specific mechanism or uniqueness of dewaxed shellac that allows all other top coats to bind to shellac ?
William Shade: I believe that shellac dissolve very easily in all organic solvents, and so when a solvent borne coating is applied, they will liquefy the shellac and wet into it. I have never worked with Shellac in my work in industrial wood coatings. I suspect that it will tend to greatly lower the performance of most standard solvent borne coatings, especially 2 component systems
Differences in marine grade vs others?
William Shade: (Assuming that they are for exterior wood applications) I think the biggest difference is the additives that are used to give adhesion to metal, or in our case to wet out, and penetrate the wood grain.
Often marine coatings are formulated to deal with the harsh environment of salt water.
There may be little difference between the resins and chemical functionality. But they are formulated for their specific application, and often marine coatings have a significantly higher film thickness than wood coatings.
Are the oils in a varnish mainly used to give color tone and help penetrate the urethane into the wood fibers ? I know ones used in these applications polymerize/harden a bit as well (via oxidation reaction of the double bonds in the carbon chains right ?)
William Shade: Alkyds could fall into this category, I see this as being a plasticizer in the coating that will allow the coating to be more flexible. In picking oils for a coating, they are selected so that they have good clarity, and typically the formulator chooses ones that does not affect the natural color tones of the wood.
These have become popular lately as lower noxious fume alternatives w/ the “oil” finish look & feel w/ better durability than just oil or wax finishes.
What are these made of?
William Shade: Not common in the industry, likely they are a mixture of nut oils and some other oil soluble plant materials
They will rely on oxidative cure, and these reactions may take weeks to months to fully cure. Likely will have little to know chemical and physical resistance.
The indefinite cure time will make it difficult to apply a coating over them with consistent and reliable adhesion properties
Last Updated By: Jeremy Finison
Original Post: Feb. 12, 2020
Last Updated: Feb. 12, 2020