Getting the job done through thick or thin

by Jim Williams
Experts give the lowdown on paint thickness—and why mils matter

In the world of professional painting, the difference between success and failure can come down to something as miniscule as the thickness of a human hair.

“When there’s a paint failure, what’s the first thing that someone checks? Yes, you’ve got it; they measure the thickness of the paint you applied,” Bob Cusumano, president of Coatings Consultants, Inc., wrote in a recent blog.

Cusumano should know. He has been immersed in the coatings industry for more than 30 years, first as a painting contractor and now as a consultant. His Florida-based consulting firm analyzes paint failures, writes specifications for various coating projects, and provides expert witness testimony. He is also the former national president of the Painting and Decorating Contractors of America, as well as the principal author of the PDCA Cost & Estimating guide.


Before we talk about paint application, it’s important to note how environmental factors can influence dry-film thickness.

Cusumano says the temperature of both the air and the substrates can definitely be factors.

“The viscosity increases when it is cold, therefore, the paint will tend to be thicker,” he said.

High humidity also works against a fresh coat of paint by reintroducing water into the incompletely dried paint film. Combine high humidity with low temperatures, and condensation develops on the freshly painted surface, which can mar the paint job.

“The opposite is true when it is hot. The viscosity will decrease and the paint film tends to be thinner. This will also tend to make the paint run.”


“DRY mil thickness is only half the equation. What you want to focus on is WET mil thickness,” said Nick Slavik, owner of Nick Slavik Painting & Restoration Co. in New Prague, MN.

Slavik is the host of Ask a Painter! Live, a weekly Facebook broadcast, and has 24 years of experience as a craftsman, painter, decorator and restorationist.

“Wet mil thickness is when the paint goes onto something, how thick it is applied,” Slavik said. “One mil is equal to one human hair, give or take. Every product has a technical data sheet that describes how thick to put it on; and they describe it by wet mil thickness.”

To figure out how thick the coating should be, Slavik recommends consulting the technical data sheet; take the listed wet mil thickness and multiply it by the percentage of solids in the paint.

“It’s less important when you’re doing exterior stuff because when you’re brushing, you’re really only going to put so much on till saturation,” he said. “But when you have something like Benjamin Moore’s ADVANCE or cabinet enamels and you’re spraying, the temptation by contractors is to give it one super-thick coat, thinking it will be durable and save a little time. But you’ll run into problems down the line.”

For example, explained Slavik, Benjamin Moore’s ADVANCE is 39% solids, and the recommended wet-film thickness is 3.6 mils.

“When it dries, multiply 3.6 x 39% and that gives you a dry-film thickness of about 1.4 mils. So really, in the end you’re laying down what is only [equivalent to] 1-1/3 human hairs. Use that for a reference when you’re thinking about coatings,” he said.

John Calderaio, exterior exposure station manager for Dow Chemical Co., offers another formula for determining proper mil thickness.

“It’s easy for paint scientists in a lab using scales, but for contractors, they should use some easy math. Calculate the square feet of a room you’re going to paint. Just multiply the height by the width of each wall, then add the walls together. A 10′ x 10′ room with standard 8′-high walls will be about 320 sq. ft. If you’re going to apply two topcoats, you’ll need to cover 640 sq. ft. Normal recommended spread rate by a manufacturer is 400–450 sq. ft. per gallon. So, if you’ve applied two coats of paint in this 10′ x 10′ room and you have leftover paint from one gallon, you’ve put it on too thin. You’ll need about 1.5 gallons to have the proper film thickness. You don’t have to be precise like a paint scientist is, but you do want to be close.”


“Everyone thinks thicker is better … more is better,” Slavik said. “But the problem, especially when you’re using waterborne alkyds and coatings that have a lot of chemistry going on, is you can actually trap solvents and things that need to evaporate down into that finish. It will either take longer to cure, it won’t cure at all, or it won’t cure to full hardness. The worst part about that is that you can have delamination; coatings peeling off, coatings not adhering well, and coatings that don’t go through the correct process. It can be a completely horrible process.”

Cusumano agrees. “I think that’s one of the biggest misconceptions; that more is better,” he said. “Permeability is lessened with excessive thickness. There are also many film defects related to film thickness. Excessive thickness can result in runs, sags and ‘mud cracking.’ However, insufficient thickness can result in erosion of the substrate, holidays, and incomplete coverage and hide.”

“I think some painters think that if it looks okay, the paint is thick enough,” said Calderaio. “But they’re only considering hide. Also, they think the natural spread rate is fine. For interiors, film thickness is not as critical as it is for exteriors. With too thin an interior film, you could see problems in bathrooms and kitchens; think poor scrub and more mildew issues. But with exterior applications, the risk is greater. So you want to protect your biggest asset.”


Cusumano says paint thickness is more of an issue in most commercial or industrial situations as opposed to residential painting.

“Although there are some exceptions, the main goal of residential painting is quality of the aesthetic appearance,” Cusumano said. “If the coverage is complete and consistent, then the main objective has been met. On many commercial and industrial projects, the main goal of painting is protection of the substrate. Therefore, coatings with greater dry-film thickness are often utilized.”

That noted, substrate still rules the day, explained Calderaio.

“Whether it’s commercial or residential, if it’s a new, unpainted surface, the best procedure is primer and two topcoats,” said Calderaio. “Metals, because of the threat of corrosion, require special maintenance coatings. These coatings must be applied at the thickness specified by their manufacturers, which are typically thicker than architectural coatings.”


Cusumano says all painters should know how to use a wet-film gauge and know how to calculate the resultant dry-film thickness of the paint.

“There are many different types of gauges to measure the dry-film thickness of coatings, depending on the type of substrate,” he said. “There are nondestructive gauges for ferrous metal (steel), nonferrous metals and other substrates such as wood, concrete and gypsum wallboard. There are also destructive tools that measure thickness including Tooke Gauges—and viewing paint cross-sections using a microscope with a reticle.”

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