Ask the Expert Question-and-Answer Archive

by Ron Joseph

June, 2003

Measuring and Specifying Dry Film Thickness

Q. Where a dry film thickness is specified for tanks painting (for eg. 500 microns), what is the standard method for measuring the paint dry film thickness? Whether the specified thickness is to be achieved all over the surface or the average of the thickness measured in a particular area can be accepted. If average paint thickness can be accepted, what is the upper and lower limits of the specified pait thickness (in percentage of specified thickness)?

A. Dry film thickness measurements are made using a variety of film thickness testers. You can find these instruments on the Internet by going to www.gardco.com, www.defelsko.com, www.elcometer.com, www.bykgardener.com, and many more. If you use any of the more sophisticated instruments, be sure to calibrate them before making your measurements.

I am not aware that a standard maximum or minimum exist. This is something that needs to be agreed upon by the specifier and the contractor. The Steel Structures Painting Council (SSPC) (www.sspc.org) sells a two volume set of standards and you might find more guidelines there. In any case, the specifier should be clear as to how many measurements should be taken and the spacing between them. For instance, on a large tank you might specify that measurements be taken every square meter.

In my opinion, if the specifier intends to have an average dry film thickness of 500 microns, then a minimum film thickness of 400 microns might be acceptable. There might not be a need to specify the maximum thickness, unless the specifier thinks that it would be harmful to the coating if the thickness exceeds a certain value.

If I were writing a specification for a tank, I would specify the absolute minimum acceptable film thickness and also an acceptable average for the entire surface. Rather than give the range in percentages, I would give the minimum and average in microns, since we know that corrosion can take place when the thickness is below a certain value.

I hope this helps.