well i know when i get porsches which are notorious for rusting near the battery tray. I use a product by rustoleum (cant remember the name) that supposedly converts the rust to primer but of course i sand the rust down first and always coat it with a good primer after ward. and in your situation since the engine is out i say take fenders off and cover windshield and respray it with something you can afford and that will lookgood/last and not rust. a good alternative to actual paint would be herculiner (for real tough easy clean up applications) or vehicle underliner.
