I may be wrong, and others can correct me if I am; but, for body panels and thinner stock items you'll want to use a different system than flux core (without shielding gas). I don't think that you'll be very happy with the results if you try to weld holes in your fenders up with the flux core. It's for thicker stock and you'll just end up making bigger holes out of the holes you already have.

What I would suggest doing is get a conversion kit for the welder. Unless it's not yours that is......

Anyway, my Lincoln Electric Weld Pak 100 originally came as a flux core system; but, I bought a conversion kit and a bottle to enable me to weld body panels and it was only about $65.00 for the kit and the bottle of C25 ran me about $60 or $70 if I remember right.

Good Luck with your project,
Dutch