To add to the POR-15 discussion, I've used it for a bunch of projects and have never been disappointed with the results.
You can put it on right over rust - the best way to prep the surface is to:
- 1) wirebrush the flaky bits off, but leave the 'pittier' stuff behind, because POR-15 needs something to grab on to
- 2) wipe with a solvent to remove dirt/grease/oil etc.
- 3) paint it on. I've found that 'dabbing' it on with those foam applicator brushes is the best way. No runs, and it flattens itself out so that there are no marks/texture. It takes 2 coats, but you get a great covering out of it.
However, the 'Regular' POR-15 (
http://www.por15.com/POR-15/productinfo/1GB/) is UV sensitive, and so it will break down and chip/wear off faster. You can topcoat it, but you'll need to use their special tie-coat primer (
http://www.por15.com/TIE-COAT-PRIMER/productinfo/TCG/) between the POR-15 bottom layer and your top coat. A benefit of this is that the primer is a weird blue-gray colour, so you can see where you've missed putting on your top coat.
If you want a one-step process, you can use BlackCote (
http://www.por15.com/BLACKCOTE/productinfo/BCG/) for a gloss finish, or Chassis Coat Black (
http://www.por15.com/CHASSIS-COAT-BLACK/productinfo/CHG/) for a satin finish.
I haven't had any experience with the hardnose paints mentioned by others in this thread, but the website says they're for primed or painted surfaces. I like the other coatings because they can go straight on to bare metal, and one of the ways POR-15 stops rust is that it doesn't dry like other paints. Your usual spray bomb stuff dries by having the solvent evaporate out of it, which leaves tons of microscopic holes for air & water, which means that any rust you didn't catch could continue, and eventually your paint will flake off. POR-15 dries some other way (let's call it voodoo magic), and forms a uniform, impermeable surface. However, it seems the hardnose stuff does this as well, and so moisture/air still can't get in.
Wow, this was WAY longer than I intended, but I hope it's helpful! Oh, all that, and I almost forgot pics of it on my Yamaha TW-200's frame.