I have a nice 82 GMC Jimmy (Blazer.) Unknown to me this vehicle had been leaking-probably for years-with the water going down from a roof seal to the floorpans with the result being bad rust on and through the floorpans. There are numerous rust holes. I am cutting out the worst of these and replaceing the steel. On some of the smaller holes I am using a product called MarineTech. This stuff sets up harder than steel and is (I feel) much better than fibreglass or Bondo. I got this stuff at a body shop supply. My mechanic friend has been using it for decades. I will eventually paint the pans with POR15.
Thanks

I spelled the name of this stuff incorrectly. It is Marine Tex. See www.marinetex.com. (http://www.marinetex.com.) Thanks.