On one of my motorized bicycle tanks I did actually fair in the seam on the tank with spot putty....it dries faster than bondo. (although I could probably just used on layer with bondo in retrospect) Then I primed and painted it with Krylon paint and never had a problem with the paint coming off. I did have a few furel stains, but I think that was due to the paint being of the flat variety more than anything.

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If it ain't broke, and you mess with it long enough, it will be.