I have used POR-15 on various stuff and it works great. Of course, its the cheap way out of a problem. If you are spending a lot of money on the restoration, go with frame-off and have it sand/media blasted. Spend at least 30 minutes with air hose getting stuff out of frame. Then tip frame over and 30 minutes more with air hose. Tip over and repeat air hose blow-out at least another couple of times. Mickmate was right about the beach under the car.
Then paint or powder coat. Eastwood has a ton of good stuff for rust encapsulation, chemical stripping and home powder coating.
If its just rusty in a few places on the frame, just scrape, wire brush area and paint. I'm not sure what the best way is to remove heavy rust from the inside of a frame. Maybe there is a "chemical dip rust removal" service in your area. This is very effective, but it sure
ain't cheap.
Anything that easily unbolts from the frame should be blasted and then finished.
I found a business that does cemetery headstones that does auto parts blasting at a very reasonable price (IE - cheap).
