Any where metal is shot and there is a hole DO NOT fill it with just bondo...cut and weld in new metal then fill imprefections.... if theres any serious rust but its not rusted through i'd recommend picking up some POR-15 treat the rust and use that. I'd also recommend this stuff called masterseries
http://www.masterseriesct.com/ That you can put that on rust or straight metal....POR-15 is only good on rust