OK I'm putting this 73 back together and I can't afford to get the bumpers re chromed at this time so what do we do. power wash out the rear and spray some rust converted in the bad spot and paint it back there. you can do this on the car and it may just save you from having to get it patched. any oil base paint will work with the converters . you can use spray bombs or just brush. under coating will dry and crack holding water in so that's out. a little paint will slow things down a lot. after all it's preservation and restoration we are trying for. so if you can't restore right now then preserve it for later.