At Walmart, in the automotive section, right next to the spray on truck box liner, is Rustoleum paint stripper in a spray can. I've used it a couple of times and it works well. Just apply it heavily and go have a soda or beer or two and let it do its thing. If it is hot and dry where you are working, you can wrap the wet surface in paper towel, spray again and then wrap the whole thing in plastic wrap. If you let it work long enough, this technique makes cleanup easier as most of the paint comes off with the paper towels. A brass brush does a good job of removing the rest. Make absolutely sure that you get every bit of it out of every nook, cranny, weld, etc. and rinse, rinse, rinse your frame inside and out with a power washer, then dry in the sun using a workstand to keep turning it so that every tube drains well. Once dry, wipe it down with a good prep solvent that will remove any remaining corrosion, wax, fingerprints, etc. Lightly sand the entire frame with 400grit sandpaper and wipe down with prep solvent again. I like to spray a rust converting primer inside the frame as well as the outside. Some come with an extension tube that makes it easy to fog it into stays and the main triangle.

If you don't mind waiting for a package in the mail, Eastwood Automotive Restoration sells top of the line strippers and rust converting primers, but the Rustoleum stuff from WalMart works fine. I've redone a number of frames that way. If you use stripper, make sure you get the entire frame to bare metal as residue can remain in any paint left behind and ruin the topcoat.

FYI, there is no reason you have to go to bare metal everywhere on the frame if the original paint is in functional shape. You can sand it with 400 grit sandpaper, lightly sanding where the paint is good and going to bare metal where necessary to remove rust or deep scratches. Wipe it down with prep solvent and give it a coat of rust converting primer, then a sandable primer. Wet sand with 1000 grit after appropriate drying and wipe it down good before shooting the color coat.