After sweeping everything up this is the only real hole we found. For a 20 year old bus I'd say that's not too bad. We cut out the hole and patched it with a piece of metal cut from the side panels removed later.
We painted on a layer of Coroseal. Coroseal is a rust restorer, through a chemical process it converts the rust into a stable substance... Or at least that's what the website says. People that know more about this stuff than me swears by it so I say, why not.
We let the Coroseal cure for 24 hours before painting the floors. We bought a gallon of Rustoleum high protective enamel paint and put on two coats.