In Part 1 of this article, we covered the definition, importance, and measurement of wet film thickness. Today, we’ll do the same for dry film thickness.
As important as wet film thickness is, “the total thickness of the film finish when dry” is equally important. This (in bold) is the definition of dry film thickness.
Simply explained, if it is too thin, its performance will suffer (poor durability and protection, improper sheen, poor UV protection, etc.). And depending on the finish, if it’s too thick, over time it may crack or become cloudy.
Calculating Dry Film Thickness
First, just for FYI purposes…
If your volume and type of work requires a LOT of ultra-fast and ultra-accurate measurements, ultrasonic coating thickness gauges do exist for wood coatings, such as the one shown below. They can range in price from a few hundred to a few thousand dollars.
The traditional and less expensive way to determine the total dry thickness of a coating is to first measure its wet film thickness then multiply the result by the percentage of solids by VOLUME. The equations looks like this:
Dry Film Thickness = Wet Film Thickness (mils) x %Volume of Solids
Note: most finish manufacturers only provide percentages of solids by weight. That said, if you’re using water-based finishes, the difference between volume and weight is usually less than a few percentage points and therefore negligible for our purposes.
However, for solvent-based finishes, you should be using only volume percentages, as the difference between volume and weight in these finishes is usually too high to disregard (up to a 20% difference in many cases).
Ok, back to our formula…
As we said, you first measure the coating’s wet film thickness, then multiply the result by the percentage of solids by volume. The resultant sum of all coats will equal the approximate total dry film thickness.
Again, manufacturers will usually provide the minimum (and sometimes maximum) recommended dry film thickness, or the equivalent number of coats. If you cannot find out what the optimal dry thickness should be for the finish you’re wanting to use, I recommend a minimum of 2.5 mils and in all cases, no more than 5mils.
To help you visualize this more easily, let’s use as an example Target Coatings EM8000 conversion varnish.
Assuming you have applied all coats at the recommended 2 mils thickness, and given the stated 34% of solids by weight, you come up with .64 mils thickness per coat. Multiply that by the number of coats (four) they used in the KCMA-type test, and the dry thickness of the topcoat equals 2.56 mils.
Finishing is one of the most exciting parts of any project for me, just as I hope it will be for you. By understanding film thickness and its importance to achieving terrific finish results, and applying that knowledge, I believe your finish quality will improve.
If you have any comments on this topic, or recommendations for future topics, please share your thoughts or read what others are saying in the comments section below. I’m here for you.
Marty Schlosser has nearly 50 years experience as a furniture designer, maker, and finisher, including a career in production-level spray finishing. Often called, “the industry’s go to expert,” Marty is a past-president of the Ottawa Woodworkers Association (and a proud recipient of their Danny Proulx Memorial Award), and the founding member of Kingston Wood Artisans, a local woodworking club in Kingston, Ontario, Canada where he and his wife reside.