Definition
A thin, acid-based primer applied to bare metal aircraft surfaces as the first coating before painting. It contains phosphoric acid and a resin binder, which chemically etch the metal and leave a thin film that improves the bond between the metal and subsequent primer or paint coats.
Plain English
A thin, acidic first coat brushed or sprayed onto bare metal so that the paint that follows will stick properly and resist corrosion.
Context Anchor
Seen in aircraft painting, corrosion-control work, and maintenance instructions for preparing bare metal surfaces before painting.
Derivation
Called a 'wash' primer because it is applied very thin — more like washing a surface with a liquid than building up a coating. The acid in it 'washes' and lightly etches the metal at the same time.
Why Pilots Care
Improper surface preparation leads to paint failure and hidden corrosion that can compromise airframe strength.
Intuition Check
Wash primer is not a cleaning wash. It is a thin chemical first coat applied after the surface has already been cleaned.
Example Sentence 1
After stripping the old paint from the wing skins, the technician applied a wash primer before the epoxy primer coat.
Example Sentence 2
After removing old paint from the wing spar, wash primer was used to protect the exposed metal.