I avoid any product that claims 'little surface preparation needed'. Having said that I am happy with the results of POR products after I grind and wire brush the surface with an electric grinder and then wash off with a metal prep etchant and a final wipe down with a prep solvent. I'm a proponent of the "belts AND suspenders" school of thought.
I have been working with POR15 for several years now and agree with FTF - the zero prep angle is just not the case - especially if the metal in question is both rusty and oily . . .
In my experience POR15 likes a clean rough surface. Be it rusty (but with loose scale removed) or sandblasted - POR15 will grab it. POR15 is best when worked into the surface with a stiff brush. The clear version will make a rusty surface look wet. As it soaks in the rough grain begins to surface. Working in a second coat will return the wet look. I stop at two coats cause it is best to avoid build up. The topcoat (which can even be just primer) must be applied while POR15 is tacky - cause once it cures nothing sticks.
If the topcoat is the finish coat (which I prefer) it important to avoid POR15 build up and to time the topcoat. If the top coat is applied to early the POR15 will out-gas creating bubbles and substrate shrinkage. The best time to topcoat is when POR15 is sticky to the touch - but none of it sticks to your finger! As counter-intuitive as it may sound - cool and damp days are your friend when POR15 is your base. When it is beastly hot - clean and prep and wait for a rainy day.
Beater . . . for an engine block I prefer Glyptal Red as the base coat and an Engine enamel topcoat baked together.
