Article: Understanding Solvent Based Finishes
In this tutorial we will discuss and compare various solvent based finishes, including nitrocellulose, acrylic, pre and post catalyzed lacquers and Conversion varnish.
The trade-off among catalyzed and non-catalyzed finishes is durability versus ease-of-use. Despite the unpleasant odor and greater environmental hazard of the solvent-based finishes, most finishers still use them. So let’s look at a comparison of these finishes.
The trade-off among catalyzed and non-catalyzed finishes is durability versus ease-of-use; there’s no ‘cure all’
Choosing a spray finish was easy 35 years ago when I began doing finishing. Basically, there was
nitrocellulose lacquer. Catalyzed finishes existed, but they weren’t widely available or widely used. Close to the same can be said for “water-white” and CAB-acrylic lacquers.
All of these finishes (and also a number of water-based finishes) are now available from most paint stores and distributors, so finishers have to make choices. Despite the unpleasant odor and greater environmental hazard of the solvent-based finishes, most readers of Woodshop News still use them. So let’s look at a comparison of these finishes.
Two broad categories
Most of these solvent-based finishes thin with lacquer thinner. The exception is conversion varnish, which thins with xylene (xylol) or an equivalent solvent supplied by the manufacturer.
Dividing the finishes by their thinners, however, is not helpful.
The most helpful division is between catalyzed and non-catalyzed. “Catalyze” refers to an acid additive that makes the finish cure properly. Catalyzed finishes are often referred to as “two-part” finishes.
Though several of the catalyzed finishes thin with lacquer thinner, they are entirely different from the non-catalyzed lacquers. You can’t make a catalyzed finish by adding the catalyst to a non-catalyzed lacquer.
The three common non-catalyzed lacquers are nitrocellulose, water-white nitrocellulose and CAB-acrylic (sometimes referred to as “CAB,” “acrylic” or “butyrate,” which is the “B” in CAB).
The primary distinction between these three lacquers is color, with nitrocellulose being the most amber, water-white being significantly less amber, and CAB-acrylic being colorless. Nitrocellulose and water-white continue to amber slightly over time, especially when exposed to sunlight.
Nitrocellulose resin by itself is colorless, but it is too hard and brittle to perform effectively as a finish. So a co-resin or modifying resin is added to make the finish.
Inexpensive maleic resin adds a distinct amber coloring to the lacquer.
Several alkyd resins add less ambering. More expensive coconut-alkyd resin adds very little ambering, and though there is no defined industry standard, lacquers sold as water-white usually have this resin added.
Lacquers can be made with only acrylic resin. This type of lacquer is crystal-clear and doesn’t discolor at all, even in sunlight (think Plexiglas). But a solely acrylic lacquer is very low in solids and difficult to spray, so it is rarely made.
Lacquers also can be made with just cellulose/acetate/ butyrate resin (CAB), but this resin, though colorless, is very expensive. So manufacturers usually combine the less expensive acrylic with CAB to get the best of both worlds: a relatively inexpensive finish (compared to CAB alone, but not to nitrocellulose lacquer) that is colorless and has a decently high build and good spraying qualities. This finish is also acceptably tough and resistant to moisture.
All three of these lacquers (nitrocellulose, water-white and CAB-acrylic) fit in the category of “evaporative” finishes, meaning they dry entirely by the evaporation of their solvent, lacquer thinner. They have the following characteristics in common:
• They redissolve with lacquer thinner and acetone.
• Very hot objects can soften (melt) these finishes and leave imprints.
• All three of these finishes can be more invisibly repaired than other finishes by dissolving or melting in new finish.
• Each of these finishes can be scratched easily by coarse objects, which makes the finishes more subject to damage on cabinets and tops, but also best for rubbing to an even sheen as is often done on high-end tabletops.
Just as with non-catalyzed finishes, there are three primary categories of catalyzed finish:
• Conversion varnish (catalyzed varnish)
• Post-catalyzed lacquer (post-cat)
• Pre-catalyzed lacquer (pre-cat)
All three are primarily “reactive” finishes, meaning the resins cross-link upon drying to create a much more durable and moisture-resistant coating than do the non-catalyzed finishes.
Conversion varnish is made with amino and alkyd resins. The two main amino resins are melamine formaldehyde (think the toughness of melamine panels) and urea formaldehyde (think the resistance of urea- or plastic-resin adhesive). The amino and alkyd resins are caused to cross-link by an acid catalyst, which is added just before spraying the finish.
Conversion varnish is one of the most resistant of all finishes to damage from coarse objects, heat and solvents, but it is subject to many application problems if not used precisely as directed.
The finish has a short pot life, short open time (you have to get all coats applied within a limited period), requires precise mixing with the catalyst, has to be applied and allowed to cure in warm (65 degrees or warmer) conditions, and has to be kept relatively thin on the wood (no more than 5 mils — thousandth of an inch — of dry thickness). Conversion varnish is also difficult to repair invisibly.
You can easily distinguish a conversion varnish from a post-catalyzed lacquer by its solvent. Conversion varnish uses xylene, toluene or high-flash naphtha, while post-cat uses lacquer thinner.
Post-catalyzed lacquer is simply conversion varnish with nitrocellulose added (by the manufacturer, not by you) to make the finish easier to use. You get a longer pot life, longer open time, better bonding between coats and easier repairability. You still have to add the acid catalyst just before spraying.
Of course, post-catalyzed lacquer isn’t quite as durable as conversion varnish because of the added nitrocellulose, but the difference isn’t great. The nitrocellulose is what makes lacquer thinner necessary for thinning.
To make a finish that is even easier to use, manufacturers add a weaker acid catalyst to post-catalyzed lacquer at the factory to make a single component finish called pre-catalyzed lacquer.
Pre-cat takes longer to reach its full cure than post-catalyzed lacquer, but it performs well and it sprays almost exactly the same as a non-catalyzed lacquer with almost as few possibilities for problems.
If you have a good distributor in your area, you may have access to a post-catalyzed lacquer that is catalyzed by the distributor just before delivery. Many shops appreciate this service to relieve them of having to do the precise mixing.
Choosing a finish
There are trade-offs in choosing among the six finishes. None is best for every situation.
If you’re finishing a dark or dark stained wood that won’t be exposed to severe abuse, the easiest-to-use and least expensive finish is nitrocellulose lacquer.
If you’re finishing an unstained white wood or a white stained (pickled) wood that won’t be exposed to severe abuse, use water-white or, better, CAB-acrylic lacquer.
If you’re finishing objects that may be subjected to abuse, such as office furniture, tabletops, cabinets or bartops, choose a catalyzed finish. The trade-off among the three is durability versus ease-of-use.
Conversion varnish is the most durable and pre-cat is the easiest to use.