[paint guy]

Originally Posted by 69Customs

I use Por15 as directed when possible removing rust and applying to bare metal with the phosphate as directed. I never spray as that is not recommended. I used it in the inner truck lid when I had the lid off, Cleaning out as much inside without taking it apart. I then poured POR15 into it with it upside down and swished the trunk lid so it came out all small drain holes in corners. WHY? Cause I can't get into sand surface rust out and paint. I wanted all inside creases to fill with it as it is a moisture curing paint. The more moisture the harder it gets. I figure it will block any air or moisture from getting to any surface rust and seal it from going any farther. Regular paint breathes and can let moisture thru.
So I figure something to seal light surface rust is better than nothing. Plus POR15 you don't have to mix like epoxy enamel.
That's why I use it and have had good results.

Short of chemically removing the rust by dipping, this is probably as good a system for mitigating rust inside a trunk lid as any that is available to a home restorer. I happen to use a different method, using either an aerosol (rust-convert" type product or a combination rust-remover/phosphate conversion coat type product that I will let dwell in inside areas, then drain. In either case, I will then spray with an aerosol can "cold galvanizing" spray paint (over 90% zinc) or pour in epoxy primer in the same manner as detailed above. Coming from the factory, the inside of deck lids had a conversion (phosphate) coating on the inside, but no primer or paint (other than unintentional overspray). most of them also had some kind of sound deadening material which of course, would attract moisture.

[69Customs]

Paint guy, Will the epoxy primer block moisture and air out as well as a moisture curing paint?

[paint guy]

The short answer is no. The long answer is it doesn't need to, as it protects steel in a different manner than a moisture-cure urethane. Moisture cure urethanes provide a very tight film when cured whereas an epoxy primer (and most other primer types)with a much higher P/B ratio(pigment to binder) tend to be somewhat porous. Most also contain some kind of zinc compound (usually zinc phosphate nowadays) which acts as a "sacrificial" pigment, expending itself to protect the steel it is coated on. And when water permeates a painted surface (and this happens every time your car sits out in the rain) a certain amount of moisture may seep all the way down to the steel substrate. If there are no contaminants for it to react with (water-soluble salts, rust, etc) the water will come back out, no harm done. A tighter film may actually repel the water, not letting it permeate at all. Eventually both types (and all other)coatings will fail. My issue with "Paint-over-rust" type products (and I've made it known on this forum before) is not so much the chemistry (moisture-cures can and do work well, my company sells alot of them) but the marketing and the chance of success for the average home-restorer. Epoxy is much safer to spray, and is virtually fool-proof assuming the substrate is clean and rust-free. MDI based paints seem to work very well when they work. But how can you tell? Some companies say remove loose rust, some say remove all rust and pretreat (why not use epoxy then?). When it fails, it can come off like powder coat (in sheets, because the film cohesion is much greater than its adhesion) I will admit for enclosed areas, it is certainly better than nothing, and probably better than what the factory did (7-stage wash/pretreatment only) and I see nothing wrong withh pouring into a deck lid as the above poster mentioned. But if I can clean the steel and spray epoxy, I will do that everytime over moisture-cures and powder. I don't have many issues with powder paint either, per se, but rather with some powder shops that don't properly clean/treat the substrate and/or undercure thier powder thru thier bake oven. Sorry, I digress.

[69Customs]

Thanks Paint guy,

That is what I needed to know. Being a rag top car, the floors are always exposed to rain, moisture. When I got mine I had two rust repair areas rear floor pans where a vert can collect moisture from not having top up and getting caught in the rain or just because they don't seal as well as the hardtops around windows. I wanted to make sure that if any water/moisture got into the car it didn't cause any damage to the floor lying under the wet carpet. So I removed interior, welded in new patch panels, stripped floors of all primer, wire brushed, sanded, etc. to bare metal, phosphate coated, then brush painted POR15. It dried so hard it was like ceramic glass and you would slide around trying to stand on it installing the carpet.
I don't worry about moisture penetrating thru to the metal floor anymore. You pull back the carpet and its just a hard/shinny as the day I put the carpet down. And that was years ago.
Works great for verts.