This article makes use of a clever title to combine two unrelated facts about wood finishing.

The first is: The thicker the finish film after all coats have dried, the better the protection for the wood against water penetration and moisture-vapor exchange.

The second is: 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 it 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 and delamination of veneer. It can also cause warping and splitting if the wetting and drying out continues long enough. Look at what happens to deck boards after a few years of wet/dry cycles.

Excessive moisture-vapor exchange leads 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. Consider wood windows and doors with many coats of paint and how they still swell so tight in the summer they stick, and they become loose and leak air in the winter when the air is drier.

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 than the finish, however, is the thickness of the film that is applied. The thicker the finish film, no matter which finish used, the better it is at keeping liquids and vapors from penetrating.

Take wax as an example. Wax is used to seal the ends of lumber. The wax is brushed on these ends and left thick. Wax is also used as a finish on small objects and as a polish over another finish.

In the first case, the wax is very effective at reducing moisture penetration because it is thick. In the latter two cases, wax is almost totally ineffective because it is so thin that moisture can find a way though with little problem.

The same is the case for oil and oil/varnish blend finishes. Even though these finishes cure by molecular crosslinking and are therefore of the reactive type, all the excess finish has to be wiped off after each coat to prevent it drying gummy on the wood. These finishes are therefore too thin to be very effective. Water penetrates through within seconds or minutes. (The claim of some suppliers that their oil/varnish blend finish protects from inside the wood is nonsense.)

In contrast to wax and oil finishes, consider epoxy-resin finishes often applied to bar tops and restaurant tables. These finishes are poured on, sometimes as thick as 1⁄4". 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 due to 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 is the upper limit with these finishes.

Creating a Level Finish
Achieving a level finish should always be your goal because the more level the finish 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 (see “Rub To Create a Great Finish,” in issue 156), but the work required can be reduced and even eliminated if you apply the finish level to begin with. In every case, except when wiping off the excess, you will improve the leveling of your finish by thinning it.

Let’s take each application method – wiping, brushing, spraying and French polishing – in turn.

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.

The common wiping finishes are oil, oil/varnish blend, wiping varnish, gel varnish and wax. (Wiping varnish is any oil-based varnish that has been thinned enough so it levels well.)

When you intend to build a thicker finish film, you usually brush or spray the finish. Brushing can leave brush marks and spraying can leave orange peel. With any hard-curing finish, including regular or polyurethane varnish, water-based finish, shellac, lacquer and catalyzed lacquer, you can reduce these flaws by thinning the finish with the appropriate thinner.

Use mineral spirits (paint thinner) with any type of varnish, denatured alcohol with shellac, and lacquer thinner with lacquer and catalyzed lacquer.
Thinning water-based finish is more complicated. You can add a little water, but this seldom corrects the problem because water has a high surface tension. It’s better to use the manufacturer’s “flow out” additive. Unfortunately, only a few manufacturers commercially supply one.

It’s easy to picture how thinning can be used to achieve total flatness. Imagine brushing or spraying just one 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. To get a good build quickly and still achieve a level end result, apply several full-strength coats, sand the surface level, then apply one or two thinned coats.

French polishing is a method of applying shellac with a cloth to achieve a perfectly flat, 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 to do the thinning 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 and add more alcohol and less shellac each time you refill the pad until you are adding only alcohol. (See “Fixing Finish With French Polish,” in issue 144.)

In summary, thicker finish films are most effective for protecting wood from water penetration and moisture-vapor exchange. Thinned finishes flow out and level better than unthinned finishes. PW