Ask the Expert Question-and-Answer Archive

by Ron Joseph

How to choose correct dry film thickness?

Q. We need to apply a salt resistant painting system on a steel structure and we would like to know how can we determine the required film thickness? The recommended system is epoxic polyamide (2 coats 8 mils each). The vendor is suggesting 2 coats of 20 mils each, but we think it is excessive. How can that difference in thickness affect the performance of the painting system? We will appreciate your help.

A. I suggest you follow the recommendations on the paint vendor's technical data sheet. If you are painting steel that will be in a moist salt environment, heavier film thickness is preferable. If the paint film is too thin moisture can penetrate the coating and diffuse to the substrate where corrosion will take place.

There is a disadvantage and potential danger to applying excessively thick coatings. If the epoxy contains solvents, the thick coating can prevent the solvents from totally evaporating and in the long term you can experience blistering and peeling. In that case you must allow sufficient time for the solvents to evaporate from the first coat before applying the second. But even then, 20 mils seems excessive.

However, if the epoxy does not contain solvents and is 100% solid, there is no danger of applying 20 mils per coat. Coatings formulated with 100% solids are designed for thick application.

Regards,

Ron Joseph