You will need a lot of POR-15 on that old bike.
The front fork appears clean and in decent shape, but the rest of the steel tubes making up the frame will need POR-15 to try to strengthen(stiffen & stabilize) the severely rusted, thin areas that without too much effort that you might be able to poke a screwdriver or icepick through the severely rusted areas.
POR-15 it, both internally and externally and you probably will be OKAY. I'd inspect and sort of test the bike frame before you do anything because if any tubing is deteriorated so badly that it crushes/bends/or breaks, you might need to cut and weld in a new solid-stable piece of similar steel tubing in place of the severely rotted section. If that is not the case, then POR-15 should be enough. Remember that you'll need to Top-Coat Over (PAINT OVER) POR-15 because I don't believe it is UV resistant, and will maybe deteriorate in direct sunlight if not top coated with ordinary paint.