Wash Primer Again?

Q. In several past issues you stated, “...never, never use a phosphoric acid activated vinyl wash primer over a pretreatment.” In theory, this makes sense. I also asked the question, “Why use phosphoric acid on something protected from its action by a pretreatment chemical film?”

But, actual service history proved me (and you) wrong. In the past several decades, the exterior finish system on USAF and US Navy aircraft was chemical pretreatment film, phosphoric acid activated vinyl wash primer, primer and topcoat. I saw too many aircraft (C-130, C-141 and so on) with that finish system that had served in every climate, including sand storms to doubt it anymore. After five and even eight years, the paint look great. Also wash primer over chemical pretreatment on test panels, aircraft parts and aircraft was exceedingly difficult to strip. After several applications of paint strippers and abrasive media spots of wash primer still remained. J. M.

A. What can I say? It’s another example of the innate perversity of inanimate objects. The reason for the caveat, “Don’t apply the aforementioned wash primer over chemical pretreatments,” is because the phosphoric acid is supposed to react with the bare metal. In theory, the vinyl resin, the acid and the metal form an organo-metallic compound at the coating to metal interface providing superior adhesion and corrosion protection to the metal. J. M., thank you for your comments.