Ask the Expert Question-and-Answer Archive

by Ron Joseph

October, 2004

Blasted Classic Car Bodyshell Protection

Q. I live in Ireland and am currently restoring a 70's bmw. I'm having real problems finding the answers to my questions about bare metal protection. Using a portable pressure blaster, i am blasting, panel at a time, the whole car. After blasting and cleaning, should i spray zinc phosphate onto the metal? Will this affect the etch-primer that my painter will be using?

When you write about pressure blasting, does that mean you are blasting with high pressure water, or are you using an abrasive,such as sand or glass beads? Second, when we speak about zinc phosphates we generally refer to a zinc phosphate conversion coating, which is a water-based inorgtanic chemical that passivates the steel from corrosion. Is this what you are doing? I need more information on what this zinc phosphate is. Finally, I assume that by "etch primer" you are referring to a conventional vinyl butyral etch primer that contains phosphoric acid?

IF you are applying a true zinc phosphate conversion coating to the bare clean steel, then you do not need to apply an etch primer, because the phosphoric acid in the etch primer will not be able to etch the steel. The zinc phosphate will already have done this for you. In fact, it would be detrimental to apply the etch primer. Instead you should apply a full bodied corrosion preventive primer, such as a zinc chromate primer. You can definitely apply a corrosion preventive primer that does not contain an acid, such as the etch primer. Thereafter you can finish with a basecoat and clearcoat, or if you are not planning on doing this you can apply a high gloss finish coat directly over the corrosion preventive primer. bare steel (or aluminum) surface and do not have an aqueous (water-based) conversion coating (such as an iron or zinc phosphate) that will passivate the steel and prevent corrosion. If the steel is already covered with ANY other coating (organic or inorganic), the acid in the etch primer will not have anything with which to react, and this surplus acid will then detrimentally affect the primer and topcoat that you will be applying thereafter. I hope this answers your question. Good luck with your BMW. Please send me a digital photo of the finished product!

Ron Joseph