I have used Hammerite and POR.
One issue I have with Hammerite is that the spray heads get plugged easily and you wind up with a half can that you can't use (they use a special head as I recall) so I suggest using the brush on version rather than aerosol.
Hammerite will flake off if you place a bolt nut or screw over it after it has dried and tighten it. It is a sort of glassy material that doesn't adhere like regular paints do, but it does stop rust, try putting it on an old rusty battery tray and you will be amazed.
POR is great for what it does, but they say you should not or can not paint over it with another paint product and that can be a bummer if you planned to hide it (its sort of a ugly pasty stuff).
http://www.por15.com/
Dieseldiehard
1971 220 (gas) 4-spd manual 102600
1979 300TD w/ ’85 turbo engine 288975
1983 300D 237300
1985 300TD 202820