I would sand blast the skins and use metal glaze for the filler, its a lot "lighter" then bondo and a heck of a lot easier to sand. It is great for filling small stuff, but it is more watery then bondo so can be sort of trick to fill bigger spots. If you decide to go that route, use 180 grit sand paper on a soft block for sanding ( get a roll of 180 for a d.a. that way you can just stick it to it )
Tyler