Advice from the discussion list 2CV-L
POR is an anhydrous (water-absorbing) coating which inhibits rust. It is an epoxy-based long chain polymer + silicon which absorbs water as it sets, like a thick paint, then seals with silicon against the rust. The compound also contains polymer-based rust inhibitors which stop the oxydation process. you can literally paint over rust, if you so desire, or sand the rust, fill dents with a silicon bonding agent and sand, then coat.
Wears like iron, and "case hardens" the area coated. Because of its characteristics, it can also, in a slightly different formula, be used on exhaust maniforlds, heater boxes, battery boxes, to reduce corrosion. Acid from batteries does not affect it, it just washes off with no effect to the area touched by the acid.
The only possible drawback to POR15 is that it must be painted over, because UV/sunlight discolors it, and makes it look ugly, but does not affect the performance. People coat undersides and don't paint, since on most cars, the sun doesn't shine there. But for use on other parts which may see sunlight or regular light, a good paint is desirable, or your car will look like it has some terrible skin disease, even though it will wear like iron.
My new galvanized chassis will be coated with the stuff, as will the bumpers, because it hardens so well that it resists stone chips considerably more than anything I've found.
Speaking of rust, I was just reminded of this today. I had taken this picture a couple of weeks ago & forgotten all about it. The picture is at: http://groups.yahoo.com/group/2CV-L/files/GeorgePics/ and is of the basketball hoop pole I put in the ground in the Spring of 1990.
Three years ago (Summer '98), when the blue thing madness began, I noticed some rust spots on the pole & thought "Aha! Good Test"! So I just wire brushed the spots & spread on some POR-15 without the recommended 20 minute detergent wash OR the 20 minute phosphate etching rinse. No prep at all. Just wire brushed.
The large circle at the top & the small circle at the bottom is POR-15. All the rest of the black is the factory paint which has been completely undermined with rust. I picked it away to show up the POR better. The POR looks almost like it did the day I put it on 3 years ago. There is no rust UNDER it & you cannot scrape it loose from the metal. It is still very strongly adhered; just as tenaciously at the edges as in the center. This pole gets whatever the weather is every day. Just no salt. Draw your own conclusions.