Back in the dark ages when I was teaching collision repair, we used a 40 grit sanding disc to bare metal before applying Bondo to give the metal some "tooth" for the Bondo to grab onto. Fill, then sand, prime, and paint. Maybe old school, but it worked well!

"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!