Do you want a painted finish or do you want a bare metal finish?
I don't know for sure but I would worry about using Bondo on something as flexible as a fender. I would think the edges may crack loose and you'd see a line around the perimeter of the repair.
As long as we are discussing Bondo, there is another auto body repair product, usually used in conjunction with a Bondo repair. It is thinner-bodied and comes pre-mixed in a tube (solvent based). Bondo isn't good for filling thin sections but this stuff is. It is sued for filling imperfections for which Bondo is inappropriate.
Last edited by Mike Mills; 12-19-10 at 05:20 PM.