I'm going to guess that after you wire wheel the floor like Kevinski suggested, you'll find some pinholes. You may want to cut those areas out and patch them, but if it's fairly sound, you could possibly seal the holes with something like JB Weld. But I would sandblast those areas at least before trying JB Weld. You'll have to use your judgement on whether to weld in new sheet metal.The floor is a weak spot for rust as that floor mat backing holds moisture.
