Ask the Expert Question-and-Answer Archive

by Ron Joseph

May, 2003

Paint Line Cleaners - Surface Pretreatment

Q. We operate a multi-stage electrostatic paint system with a spray cleaning stage for removing manufacturing oils prior to spray substrate application, painting, and oven cure. We are applying an epoxy paint to steel. Can paint "thickness" have an affect on "paint adhesion" when evaluating new cleansers for the cleaning stage? Please note that in performing our ASTM cross hatch adhesion test, we do so "after" the ASTM B117 salt spray test (370 hours). Thank you.

A. Film thickness of the paint system can have an affect on the adhesion of the entire coating system to the substrate. If you intend to evaluate different chemical cleaners and perform adhesion tests after the salt spray test, then insure that you compare the adhesion of the coating to the substrate when all painted panels are identical. That means that the coatings, intercoating times, film thicknesses, etc., are the same.

Alternatively, you can evaluate the cleaners by applying only the primer and maintaining its film thickness for all cleaners. Thsi method might be easier to control as you can eliminate some of the paint application parameters.

Followup Q. I am a bit puzzeled though. Are you saying not to paint the parts to eliminate paint thickness as a variable? If so, then how do I evaluate the effectiveness of the cleaning chemicals? My line of thinking is that paint and substrate will not stick to dirty parts. Therefore, to evaluate the effectiveness of a new cleanser I have to do an adhesion test on the paint.

A. Yes, that is true, but are you evaluating the cleaners for their ability to clean the metal or for corrosion resistance. If you want to check the adhesion only, then by all mean apply all of the coatings. If you are using ASTM B117 primarily to evaluate the cleaners for their ability to protect the substrate from corrosion, then you need only apply the primer.

If you are only interested in adhesion, then you can apply all of the coatings to the panels, let them condition for 96 hours, drop them into a bucket of water for 24 hours and then perform the adhesion test.

I don't have an objection to doing what you suggested in your first email, but insure that the parameters are controlled.