If you are in an area where there are marine supply stores, buy a quart of Pettit's Rust-Lok Metal Primer. Seems like pretty much the same stuff as POR-15 for a lot less money. Coal-tar epoxy , sold as Z-Spar's Poxitar at marine supply stores, over the Rust-Lok makes for a good suspenders-and-belt system. I've also been using Zero-Rust, which seems to work just as well as POR-15 or Rust-Lok, costs about the same as Rust Lok, but has a longer pot-life. So you waste less. Two-part epoxy putty, from POR-15 or the Home Depot On Line Store, works well on pin holes; so does the very thin fiberglass matt that POR-15 sells (ipd also carries their products), but I haven't found another source for the fiberglass. You first treat the rusted area with POR-15 or Rust-Lok, let the final coat get tacky, lay down the fiberglass, and then daub on several coats of POR-15 or Rust-Lok. If you then apply Patrick's roofing tar and George's rooffing felt over that , the floor should last for a while. Nothing, of course, beats welding in new sheet metal.
Bob S.