Ask the Expert Question-and-Answer Archive

by Ron Joseph

October, 2000

Painting Galvanised Steel

Q. We have a typical problem coating new galvanising. In one source of galvanised structure the PVB wash primer is reacting and exhibiting good adhesion of subsequent coats but in another source of the structure there appears to no reaction of the etch primer and as a result all the coats are delaminating from the substrate without any adhesion. My questions is how to classify different grades of galvanised steel. I beleive there are two methods of galvanising steel: What are they and how each one of them affect coating adhesion. Please advise and forward any information that you have on coating galvanised steel.

A. The Polyvinylbutaryl (PVB) wash primer contains phosphoric acid which does the etching of the galvanizing (zinc). I'm don't know much about different grades of galvanizing, but I do know that some are post treated with a temporary protective clear (or amber-colored) coating to prevent the galvanizing from corroding during storage or shipment. This coating must be removed before the wash primer can be applied. Removal can be easy or difficult, depending on the type of clear coating used, and your vendor should be able to tell you how to remove it.