Ask the Expert Question-and-Answer Archive

by Ron Joseph

November, 2003

Measuring Dry Film Thickness on Aluminum Sand Castings

Q. A government audit is requiring variable data on the dry film thicknesses of the paint used on various purchased components provided by our vendors. The paint and primer have mil specs which our vendors validated and certified, but there is no mil spec called out on the prints for performing dry film thickness measurement. Different vendors used varying techniques from deriving thickness from the wet mil measurement to coupons being optically measured. Is there a best practice? What is the best means to non-destructively validate dry mil thickness on a finished part.

A. Do you need to know the composite (total) dry film thickness for the coating system, or the film thickness for each of the coatings (primer + topcoat) in the system? What are the substrates? What was the surface preparation?

My preference would be to have primer and paint thickness separate, but total dry film thickness would at least give me some variable data. The substrate is A356.0 Sand Cast Aluminum. The majority of these parts are machined castings. I have no information on surface preparation.

A. You can purchase a nondestructive dry film thickness gauge to measure the total film thickness of the coatings. The gauge must be designed for nonferrous substrates, and you can buy instruments that do both ferrous and nonferrous interchangeably.

Because you are measuring over a casting you will need to perform frequent calibrations, but the instruments come with calibration shims.

If you need to measure the individual coats of paint you will need to do this destructively.