Just like the fuel tank the bottom of the car above and around the tank had been undercoated. And just like the fuel tank the undercoating had begun to fail. However the tank was tin-plated and resisted corrosion better than the steel under the car. So this got to all be scraped off too. This was as much fun as scraping the tank with the added joy of having to lay underneath the falling crap I was scraping off. I was picking black gunk out of my hair, eyes, and ears for a week. I tried to use a drill with a wire-brush attachment to remove the mess but that only worked where the undercoating was gone.
Here you can see an area where the undercoating was just falling off with surface rust underneath. I quickly found that a little heat from a hair dryer helped release the undercoating from the metal.
Here was the end result of all the cleaning. At this point I had scraped, wire-brushed, cleaned with kerosene, cleaned with a strong detergent, and finally rinsed. I started cleaning at the main frame cross-member right behind the rear axle (at the bottom of the picture) and cleaned right up to the back of the car. I eventually removed the rear bumper for better access there. The fuel tank mounts between the two cross-member you see in the picture.
Here is a shot of the primer drying. The arch in the body that you see is where the fuel tank filler neck goes over the frame rail. I had used a brush on the fuel tank and was not really happy with all the brush marks. Also a brush is a pain to use overhead. So I bought a pack of mini foam paint rollers and used that to apply the paint. It worked great on the big flat areas but all the nooks and crannies still needed a brush.
Here is the a similar shot after painting with flat black. Again the roller left a much nicer finish than the brush did on the tank.
After a few days for the paint to harden I undercoated everything with a spray-can undercoat. This is right after my first coat. It was still very wet and shiny but dried to nice rubbery matte black. It went on very easily but I did get some overspray on the painted surface of the car. Lucky for me the paint has been seriously waxed and the overspray came off pretty easily.