You may not have thought of it this way, but the opposite meanings of the words "thick" and "thin" explain a lot about wood finishing.
The thicker the finish film is after all coats have dried, the better the protection for wood against water penetration and moisture-vapor exchange.
On the other hand, the more you thin a finish, the better it lays out flat. That is, the thinner the finish you are applying (which is not the same as the thinner you apply the finish to the wood), the more reduced the brush marks, orange peel and rag tracks.
Protecting against moisture
A finish has two functions. The obvious one is to improve the appearance of the wood. The more important one is to protect the wood from water absorption and moisture-vapor exchange.
Water absorption causes black staining while causing finishes to peel and veneers to delaminate. Repeated cycles of wetting and drying can also cause warping and splitting if the cycles continue long enough - the common exterior example being boards in a deck after a few years of wet/dry cycles.
Excessive moisture-vapor exchange on interior furniture and cabinets can lead to joints breaking down sooner because of increased shrinkage and swelling in the cross-grain construction.
No finish totally stops the passage of moisture in vapor form (humidity). Finishes merely slow the passage. The best proof is found in wood windows and exterior doors with many coats of paint. In the spring and summer, the windows and doors swell so tight you often have difficulty getting them open. In the winter, when the air is dryer, they become loose and leak air.
Reactive finishes (varnish and catalyzed finishes) are better at slowing water penetration and moisture-vapor exchange than evaporative finishes (shellac and lacquer) and coalescing finishes (water-based finish). Far more important, though, than the type of finish, is the thickness of the finish film. The thicker it is, no matter which finish is used, the more resistant it is to the penetration of liquids and vapors.
Maybe the most extreme example of the importance of thickness is the contrasting protection wax provides when brushed and left thick on the ends of boards to prevent splits and checks versus the almost total ineffectiveness of wax as a moisture barrier when used as a polish over another finish.
Liquid moisture might bead on the waxed surface because of wax's low surface tension, but moisture in both liquid and vapor form penetrate almost as if nothing were there.
The same is the case for oil and oil/varnish blend finishes. Even though these finishes are of the reactive type just like varnish and catalyzed finish, they don't cure hard, so all the excess finish has to be wiped off after each coat. The resulting film, even after many coats, is therefore too thin to provide an effective moisture barrier for longer than a few minutes.
In contrast to wax and oil finishes, consider epoxy-resin finishes, which are often applied to bar tops and restaurant tables. These finishes are poured on, sometimes as thick as a quarter-inch. They are so effective at reducing moisture-vapor exchange that boards can be assembled in butt and miter configurations without fear of the boards breaking apart because of cross-grain swelling and shrinking.
One important caveat when it comes to thickness is that catalyzed finishes tend to crack if applied too thick. Three or four coats (four or five mils) are the upper limit with these finishes.
Creating a level finish
Achieving a level finish should always be your goal because the more level it is, the better it looks and feels. Of course, you can always make a finish level by sanding it after all the coats have been applied, but this is a lot of work and the need to level can be reduced or even eliminated if you apply the finish level at the start. In every case - except when wiping off the excess - you will improve the leveling of your finish by thinning it.
Consider each of the four application methods: wiping, brushing, spraying and French polishing (counting this as separate from wiping).
In most cases, the reason you apply a finish by wiping is because you intend to wipe off the excess. If you do this, you will always achieve perfect levelness as long as you have prepared the wood well and you get the finish wiped off before it sets up too hard. There's no need to thin the finish except to increase the amount of time you have to wipe off.
When you intend to build a thicker finish film, you usually brush or spray the finish. Brushing leaves brush marks and spraying leaves orange peel. With any hard-curing finish, including alkyd or polyurethane varnish, water-based finish, shellac, lacquer, catalyzed lacquer or conversion varnish, you can reduce these flaws by thinning with the appropriate thinner.
Use mineral spirits (paint thinner) with any type of varnish, denatured alcohol with shellac, lacquer thinner with lacquer and catalyzed lacquer, and xylene or a manufacturer's proprietary thinner with conversion varnish.
Thinning water-based finish is a little more complicated. You can add a little water, but if you need to thin a lot, you should use the manufacturer's flow additive.
It's easy to picture how thinning can be used to achieve total flatness. Imagine brushing or spraying any of the thinners onto wood. The thinner will level out perfectly, of course. It's only logical, therefore, that a finish can be made to level perfectly somewhere between full strength and no strength - that is, just the thinner.
The downside of thinning is that you reduce the build of each coat. A trick for getting a good build quickly and still achieving a level end result is to apply several full-strength coats, sand the surface level, then apply one or two thinned coats.
The concept of thinning to achieve better flatness is true even with French polishing.
French polishing is a method of applying shellac with a cloth to achieve a perfectly flat and high-gloss finish. The cloth is made into a pad and the shellac wiped on, often in circles or figure-eights and usually with the aid of mineral oil to lubricate the rubbing.
Similar to the brush marks left by brushing, the pad will leave rag tracks if the shellac is too thick. The way to achieve a perfectly flat French-polished surface, therefore, is to begin thinning the shellac as you proceed through the final applications.
The most efficient way of doing this is right on the pad. After you have built the thickness you want, begin adding alcohol to the pad along with the shellac. Use two dispensers, one with shellac and the other with alcohol, and add more alcohol and less shellac each time you refill the pad until you are adding only alcohol.
In summary, thicker finish films are more effective for protecting wood from water penetration and moisture-vapor exchange. Thinned coats of finish flow out and level better than unthinned coats.
Bob Flexner is author of "Understanding Wood Finishing."
This article originally appeared in the July 2010 issue.