Ha, I would see this thread as I'm rebuilding/repainting my 550F...
POR-15 CAN go over previously painted surfaces, but with a little work. On the first bike I used it on (TW-200), I cleaned the frame really well with Simple Green and plastic & brass bristle brushes. I wiped it down with solvent for painted surfaces (forget the name, but it was made by Du-Pont). Then I used a foam applicator to 'dab' on the POR-15 tie coat primer, which is self levelling. Finally, I gave it a two coats of Chassis Coat Black, which is a black, UV resistant semi-gloss covering. Now, ~3 years later, the only areas which have flaked or worn off are the 'high-wear' areas -- top of the frame under the seat, part of the footpeg my boot rubs against, etc.
With the 550, I decided that I only want to have to repaint the bike once, and be able to do touch-ups myself if/as necessary. I sandblasted the frame, again wiped it down with solvent, and then used POR's 'metal-ready' prep. This is less of a pain in the ass then you would think. It only takes 15 minutes of contact, which I accomplished by dabbing it on everywhere (again, with a foam applicator). The directions call for the metal ready to be washed off afterwards, so I sprayed the frame down with a hose and left it in the sun for a day, then next to the furnace overnight. Then, it took me two days to get the frame painted with 'Black Cote'. This was mostly because I couldn't paint areas that the frame was sitting on, and it needs about a day to harden enough that you can flip it over without scratching/rubbing off your hard work from the previous day!
A pint will be fine for the frame, you'll have about half of it or so left over. I've found that stirring the whole can, then using a coffee scoop to transfer the paint to a yogurt container/plastic cup/etc. works well. POR-15 actually dries by taking in moisture from the air, somehow, so if you paint from the can, you won't be doing yourself any favours. Once you've transferred the amount of paint you need, a piece of saran wrap between the can & lid helps keep out air/moisture.