I have used POR 15 many times on fuel tanks, floors etc.. Most important thing is to make sure you clean all grease, dirt, paint and scaly rust, then degrease the area, rinse well and dry. Next step is to use an acid etch to give the surface a tooth for the POR to bite to. Rinse again and let dry. If there are any small rust holes you can use masking tape on the outside and POR will fill fill them in. If you can reach in the tank a brush will work fine. Two coats will work best.
I hope I explained it's use well enough. There is a learning curve to its use. The product does exactly what it says it will do as long as the first steps are followed.