Undercoat
I would remove as much of the loose rust and debris as possible. Use rust encapsulator on the rusty places. That turns the rust into something other than rust. Then go to town. If you have an air compressor you can spray it cheaper than getting it by the can. I buy it by the gallon at my paint supplier.
Thanks. I am working on a 75 bronco that was a rust bucket. Doing that really is working out well for me. If you get the rust encapsulator in the paint can you can use paint thinner so you can spray it too. When it dries it looks like frame paint. I bought a load of it off eBay for a better price than Eastwood.

