if it is really big and rusted all the way through you may wanna fibreglass it then bondo over it... if you just bondo it lots of times it does crack... actually now that i thought about it for a second... they actually sell a fibreglass based bondo, it may be a good idea to try that. but make sure you sand ALL of the rust off of the metal... then bondo/fibreglass it, then seal it with primer(can of spray paint), then primer sealer(can of spary paint)... then you get it painted