This should net you about 7yrs on the floor:
Use a grinder and get all the scally rust off. Do underside as well.
Use some Marine clean to wash it down and scrub with a new bbq brush. Rinse very well, inc brush.
Use Metal ready and scrub again then rinse clean. Be sure to rinse everything well.
Use rags to mop up puddle areas and let dry a day or till it's DRY!
Put a coat of POR15 on and let it get tacky. Then get some fine glass matt or use the trunk repair kit from POR, And stick it down and give it a coat, which I now called my first coat. (since this is the first coat on the new fiberglass floor.) Let it dry for a day or two and once hard rough it with a scottch brite pad every where.
Give 2 more coats (drying til tachy not able to see on your fingure but tacky) and proceed to underside of floor pan, and let completely dry day or two. Turns hard like steel and works great.
I have done this twice now once 5yrs ago and last year, since the first one holds so well and was still holding when I sold the rig.
Just follow the POR instructions, it works.
I have done more on my rig in regaurds to the addation of B-quiet, a sound deadening / heat elimination and also a heavy Jute underlay.
I figure if I get 7yrs out of it, I then could do floor pans welded in at that time. I may by a mig by then.
Hope that might help.