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It depends what you want to do with the truck. If you're going for a show truck and perfection is important, there are stamped floor pans available for replacement (if my memory serves me right). This is also what you might have to do if there are large gaping holes or its structurally unsound.
If you're building a driver like I did, and you are dealing with smaller rust areas here's a suggestion. Go to your local auto parts store and buy Fibre-Strand body putty. Basically bondo with fiberglass strands and does wonders at reinforcing sheetmetal where a perfect finish is not required.
Clean the floor well of all rust and scale, mix and apply as directed on the can. When it hardens I used a 4" grinder to knock down the high areas and even duplicate the original contours. Use multiple layers as necessary. When primed and painted and covered with carpet or floor mat, it is solid and still looks stock.
I don't know what your end goal is but you really need a welder, or access to one, to restore an old truck body and have it last. A MIG welder is the best choice that is somewhat affordable. Any method of repair that does not remove the damaged metal and replace it will be temporary. Bondo is best reserved for filling those shallow dents you can't work out with the body hammer. If you have no choice but bondo, then seal it on the backside with undercoating to slow the deterioration. Plastic filler itself is fairly sturdy. But it draws moisture which attacks the surrounding sheetmetal in a hurry. Good luck.
Thanks for all of the ideas for repairing of my 50 F1 floor boards. At this point, even though the truck is pretty much stock and complete and lots of potential, I only plan to use it as a occasional driver (I needed a truck and didn't want to buy a newer model).
As an alternative to Fibre-strand, has anyone ever tried fiber-glass resin with cloth?
Replacement floor pans are available for the cab if you want it to be like original. I replaced the entire floor in my 48 by purchasing a sheet of 18 gauge steel,making a pattern out of cardboard to fit the foor, and cut the sheet to size. After it was all welded in place I seam sealed it and painted it. Worked great!
Fiberglass with resin is great. It sands like concrete and the sanding dust feels really good when you get it on your sweaty skin. You know, that feeling you get when you add another layer of insulation in the attic. Seriously though Ron, some people really like it. It is stronger than bondo and doesn't absorb moisture. I don't personally care for the stuff. It will still rust back around the edges unless you remove all the rust and get it sealed well when you are done.
POR-15 makes a neat kit that is called something like Trunk Reapir kit or something. It includes some POR-15 to and fiberglass mesh, resin and some other goodies to fix up holey floors. I've used other stuff from them and love their stuff. I plan to use the trunk repair kit on my Vista Cruiser for some holes in the rear cargo area.
Gary M.
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(2)46 1/2 tons-1 daily driver
47 1/2 ton-project truck
57 Ford FL 500-4 dr-312/AT
70 Mustang SportsRoof-351W/AT
72 Olds Vista Cruiser-350/AT
88 Bronco II, 500,000+ mi.