Here is what I did and I'm not saying it is the best way or even correct, but it is what I did on my frame.
POR-15 (click here) POR-15 FAQ'S
after the frame was sandblasted. Follow the instructions and do not get any water/moisture mixed in the paint as it will ruin the paint. Wear gloves. If it gets on your skin, it has to wear off - ask me how I know. Wear clothes you can toss out should you get any on the clothes. Great stuff, but pricey.
I only bought what I needed at a time in quarts, not the gallon. I did my frame in 3 sections, rear, middle, front when I did mine. So I used a quart for each section, BUT, I did not put on two coats as the directions say. I used 1 coat because my frame cleaned up well and I did not have any heavy rust on it. Did not do any other prep either after I sandblasted the frame other than blowing off all the sand/dust. I let it dry real well for a good week, then scuffed the paint and used the POR-15 top coat, then let it dry for a couple weeks while working on the frame - suspension, brakes, lines, etc.. Then I scuffed up the POR-15 and did 1 final top coat using Rustoleum black. Caution: if you put too heavy a wet coat of final Rustoleum paint on the POR-15, it can "crackle" the POR-15 paint. I had one area that did this, but I think it was more due to the fact that I painted it too soon and all the solvents had not dried out of the POR-15. Wasn't a problem for me as I simply scuffed up the paint and put down another coat on top. Not perfect, not a show car, just want my frame protected to last.
To do the inside of the boxed frame areas, use the Eastwood Internal Frame Coating 14oz Aerosol Eastwood Internal Frame Coating 14oz Aerosol
This has the nozzle extension and gets inside the boxed areas really well. Again, not inexpensive, but it does the job. I went with the green zinc. Anything that spays/drips out the frame holes can be painted over with your black paint. I boxed in my side rails, so needed more than if yours are open. I used 3 cans, so you could probably get away with 2 cans.
Lot of info on the POR-15 site and you can order it from several places. I see you can even order it from your local Walmart which seems a little cheaper than others and might save you on shipping. Eastwood has their version of rust inhibitors as well, but have not used them. There are other options I am sure, and others will chime in and give you more choices.