Painting the trunks/engine bay/underside/cabin, POR-15 and leave black? Options

[boxsterfan]

I have been thinking about what I will do with my car in the future when it comes time to restore. On many cars, I see people have appeared to POR-15 (or other) the trunks/engine bay/underside/cabin and then left it black? They then go on to paint the rest of the car as normal (orange, red, white, green, etc...)

I'm not looking to build a concourse winner myself. I am looking to build a car that is restored and protected from rust for as long as possible (another 40 years lets say).

What do people think of POR-15'ing (or any other product) the trunks/engine bay/underside/cabin and then leaving those areas as that? And then (in my case blue) painting the rest of the car as normal?

[Madswede]

Hmmm. If what I know about PR-15 is true, any time it peels or flakes off would be due to improper bonding to the steel underneath. Several things could cause this I suppose, but it would seem most likely that preparation work was insufficient.

Most commonly organic oils remaining within the first few microns of the surface of the steel would do it, which is why one needs to wash it pretty damn thoroughly with a good degreasing soap. Also, the rust could have been already either flaking off or about to, and not structurally sound enough to hold its own weight. The prep work is time consuming!

The chemistry of POR-15 is such that moisture actually helps the curing process. The correct metal oxides (not just the ferrous or iron oxides that is the bulk of what we know as rust, but zinc phosphates) seem to be the major contributor to its bonding chemistry, and is also why they have a specific prep chemical (essentially phosphoric acid - I dunno about zinc content, but it appears to be needed in some amount, probably very little though).

I'll let everyone know in 10 years or so how my car's application is doing. I'm not trying to brag, I sincerely hope the guys behind the technology are correct and some people's negative experiences are the exceptions, not the rule. Just for what it's worth.

By the way, the few bits I got on my skin (wrist/forearm area) came off after a few days once I shed those skin cells as normal people do. I don't think it's meant to be a top coat - they do say it should be coated with a clear top coat to protect from UV - but regardless, I noticed the same issues with finish. Kinda chaotic. Then again, we were only doing the bottom of the car and some of the engine bay.