SOLIDS CONTENT AND FILM THICKNESS
Most wood finishers measure the thickness of their finishes by the number of coats they apply. Though measuring film thickness this way is easy, it's not very accurate - for two reasons.
First, finishers differ in what they call a "coat". Some finishers define a coat as one pass, with each stroke overlapping the previous stroke by half. Other finishers define one coat as a tack coat followed by a fully wet coat in which each stroke overlaps by two-thirds. Clearly, the second group of finishers applies more finish, and thus more film build. with each coat.
Second, the solids content of the finish impacts the film build. Solids content is the percentage of solid material in a can relative to the volatiles, or solvent, which evaporate. One coat of a finish with a high solids content will leave more film build on the wood's surface than one coat of a finish with a low solids content when both are applied in the same manner.
Importance of Knowing Film Build
No matter how you define a coat and no matter what the solids content of the finish you're using, you will achieve more consistent results and have fewer finish failures if you know how to determine film build. Without this knowledge, the appearance of the finish on the wood may vary anytime you switch types or brands of finish, or anytime more than one finisher is involved in spraying. The performance of the finish may also be affected, Too thin a film build may not offer the protection required by the client. Too thick a film build may crack prematurely, especially if the finish is catalyzed.
Knowing how to measure film build can also save you money. You may be able to adjust your spraying technique, or the type or brand of finish, so that you can achieve the same film build with fewer coats. This will definitely save on labor costs, and may save on material costs.
Measuring Film Build
To get an approximation of the film thickness you are applying you need to know two things: the solids content by volume of the finish you're applying, and the thickness of your average coat while it is still wet.
The solids content of the finish will be supplied by the manufacturer. Solids content is measured in two ways: by weight and by volume, and they are not the same. Weight is the measurement manufacturers use when mixing ingredients; volume tells you what is left on the wood's surface after the finish dries. You need to know volume Your supplier can provide you with volume solids information.
You also need to know the wet thickness of the finish you are applying. You can determine this accurately by placing a mil-thickness gauge on the finish Just after you spray it. A mil-thickncss gauge in a small, flat sheet of metal with teeth cut into it at one-mil steps. (One-mil is one-thousandth of an inch.) The thickness of the film is equal to the highest numbered mil tooth that leaves an impression in the wet finish. It's best to perform this test on scrap wood. Spray the wood in your normal manner and then test immediately with the mil gauge.
A less accurate method of determining mil thickness is by appearance. A dry-looking film is 2 mils or less. A good wet film is about four mils. A wrinkled or wavy film is 6 mils or more. To then determine the thickness of each coat of cured film, multiply the mil thickness of the wet coat by the volume solids content of the finish you're using.
For example, to figure the mil thickness of a postcatalyzed finish with 35% solids by volume and applied four mils thick, multiply 4 times 35 to get 1.4 mils. It will take three coats of this finish to achieve a total dry film thickness of approximately four mils. (You usually want the total dry film thickness of your finish to be between 2.5 mils, which looks thin on the wood, and 5 mils, more than which a catalyzed finish is likely to crack.)
To figure the mil thickness of nitrocellulose lacquer with a volume solids content of 20%, thinned with 10% lacquer thinner, and applied 4 mils thick, multiply 4 times .18 (.20 minus .02) to give .72 mils. It will take five to six coats to achieve a total film thickness of approximately 4 mils.
Be sure to figure in the sealer coat if you use a separate product, and keep in mind that neither the transfer efficiency of the spray-gun type (conventional, HVLP, airiess, air-assisted airless), nor the tip size affect the results because you are measuring what has actually been put on the wood.
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