Nobody wants to hear this but, POR15, rust bullet, etc. wont do anything for you. I have repaired thousands of rust holes in vehicles, many of them that were already "fixed" with rust converter and single stage coatings like POR15. The only way to fix rust in a way that will last is to remove it and weld in new pieces of metal. Lightly rusted areas can be sandblasted until they are 100% steel, no dark rust spots or pits.
Your seatbelt area would be straighforward. Just cut out a rectangle bigger than the rusted area and weld in a new piece with the doubler plate on it. The sections where the rust is showing on the rockers are areas where two to four layers are coming together. The 80s tend to rust from the inside-out in these areas. If you don't feel like fully taking it apart and fixing it properly at least cut the outer layers back and weld in a good patch and use something that will give you a fighting chance such as catalyzed products used in the collision repair and high end restoration industries. The inside of the repair should be coated with something that will seal off all oxygen and water from the already rusted parts. Spray oil, lanolin, or wax work well, paints like por15 or any kind of aerosol do not.
