Depending on how much rot there is....
If it's just a few small holes you could just hit it with a small sanding disk and clean off the crusty stuff and patch with fiberglass resin.
When you have bigger areas of rot I would cut it out the best you can and patch with metal. Floor pan sections are not that expensive. You can trim off what you need from the new section and glue them down.
Also depends on how much humity there is by you, any exposed metal will rust that's why you need to seal any gaps with seam sealer.
Even when you use products to treat any rusted areas, it depends how rusted it is. Makes no sense to treat rusted metal if is is so thin you can poke your fingers right through it.