I've been using POR-15 for 4 years now. One of the big advantages besides the rust-prevention is its durability. Once it dries and cures you can take a hammer to it. They now have a semi-gloss black available. I'll be getting some at the swap meet next month. For a frame, I would figure a bit more than a pint. You also need the cleaner and the surface prep to do the job right, unless you sandblast the frame first. It can be sprayed, but you need NIOSH equipment (external air supply) or you will ruin your lungs.
To extend the storage life, keep it in the fridge. If I were doing a frame I and had it available locally I would buy a pint and then see how much more I needed after I ran out so I didn't buy too much.
http://www.por-15.com/