Since the sheet metal we're dealing with is either 60-something years old, or new stuff that comes from unknown places and might have whatever "paint" that might be handy at the time on it, Applying more primer and/or finish paint over that unknown layer on top of the metal underneath means exhibiting a degree of faith in the bond it has to the metal underneath is a risk I was never willing to take. "Your mileage may vary!"
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!