If you aren't willing to do it right (cut out the bad, weld, prep, paint) then either live with the rust till you can't stand it anymore and replacing the panels looks like $1500 worth of work. Or grind the crap out, fiberglas and band aid any open spots, fill and spray bomb. You will have to repeat that process every year or two as the bondo soaks up moisture and fails, but at least you saved $1500. Or just let it go and ignore the halo of rust stains that form on the driveway under the car.