I first used it several years ago (about 6 or 7 years), and very impressed at how it has held up, even some rusty frame areas I painted right over (after removing loose/flakey as instructed).
Only problem I have read on it is people in coastal areas having failures. Whatever the formula is, it reacts with salts until cured. The "salt air" near the oceans is enough to spoil a job. The instructions even say to be careful, that even one drop of sweat falling into the paint can will ruin the batch.
They have small (4oz) cans, I usually keep a few of them on hand (order a couple of six-packs at a time. The stuff doesn't keep if you try to reseal an opened can. They have quarts, gallons, etc for big projects.
Order it direct from them http://www.por15.com
You want it to be fresh.