Heat it up with a Propane torch, and scrub it with a steel-bristle brush. Above about 400 degrees F., the Bondo decomposes. That's also the easy way to remove it from sheet metal- - - -heat the back side of the panel a little, and whole chunks of it will peel off with a putty knife.
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!