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?

P- paint O- over R- rust I thingk the product needs to have a chemical reaction with rust to bond properly.

I'm not sure but I think unless the metal has been sand blasted the product will have adhesion issues on clean bare steel.

While prepping a friends 914 race car we actually wet down the bare metal after stripping and allowed it to start surface rusting before we painted over with POR.
We had been told it would help with adhesion.
The POR we use was gray and we did not paint over it, that was a mistake. After several weeks the areas that were routinely exposed to sun started turning a greenish tint.

The 914 I just bought had been stripped to bare metal and repainted. The engine bay is painted black, trunks are black bed liner and the car is dark blue, Majik Blue Pearl to be exact. I love the combination.

Jim

It is my understanding that POR-15 is very susceptible to degradation by UV light. So unless you are sure the part will never see the light of day, then you should paint over it. Yes it is unlikely that the trunk will get a lot of sun in it while you are driving. But I don't think that the POR instructions give an acceptable UV exposure time before it starts to degrade. If that point could be reached by leaving your trunk open for a few hours it just doesn't seem worth going through all the work to apply and then leave it exposed to the very thing (UV light) that could case it to fail.

I have used the entire Por-15 system inside the car and underneath. I treated the surfaces inside the uni-body and everything else with phosphoric acid (10-15% solution; aka metal etch) which converts the rust to an oxide; where I could, wire brushed and sanded treating the areas a couple of times so any rust is inserted and could not flake. Follow the instructions completely and carefully especially when you are going to paint over it. Por-15 is NOT photovoltaic friendly. You do need to use paint in areas where sunlight can get to it. Underneath the car you may not have to since it is almost always not exposed to sun rays. The same may be said for the cockpit area and the trunks; if in doubt, use paint over the Por-15.

Por-15 housekeeping is a different story which means keep everything as clean as possible; wear gloves, mask, old clothing, etc. It seems to get over everything including including the personal items. If split, clean up immediately since once it drys it develops a hard coat. Just take your time and do not get in a hurry.

IF I WERE to do it again, I would wire brush/sand areas down where I could and flush inside the unibody where I could to eliminate as much collected rust and convert it to metal oxide. I would flush and treat several times to ensure any possible rust areas are covered.

For the areas that can be painted get a good primer sealer and paint over it once applied. The trick to reoccurring rust is not just covering it with such items as Por-15, but is to first get areas converted to metal oxide and then seal,paint,etc. You do not want to have any rust to be in a flaking state.

As far as the insides of the unibody, there are holes you can go through to put phosphoric acid solution through, wash out with water (several times on each of the foregoing), and then put prevention through them. Years ago I got this information from Sir Andy but unfortunately it was stolen from me via Barstow. It is somewhere in the threads.

The system I would use is the wax one which I believe comes from Europe and was the method used for years to get preventative chemicals inside the unibody/frame areas. You pump warm/hot wax as the carrier into all the enclosed areas with the idea it will carry the needed chemicals to all the areas needing treatment. You would have to do another search to find out how it is done and where to possibly get it.

Take your time and have fun.

Steve

If you are talking about painting over existing paint, don't. It will peel after a couple of years.

Rick is right.

It is meant in most cases to be painted over. Call them if you have questions. They are very informative.

Rust is an oxide. What their piant does is covert the oxide into a protective layer that helps prevent further oxidation.

The idea behind the phosphoric acid is to etch "new" or bare carbon steel so that the surface has "teeth" and then you apply the product. If this isn't used on "raw" metal or un-oxidized carbon steel....it will peel.

The same thing will happen to powder paint on bare carbon steel if it is not de-greased and then washed with a solution of phosphoric acid to the metal prior to powder coating.

During restoration by the PO, the shop cleaned the underside and applied POR-15. I cannot not attest to the quality of the prep work, though the other body work was decent. But I can say the underside started peeling within 6 or 7 years. Guess I should add that the car is garage kept and not driven in rain, snow, etc. so it's not as if the bottom has been exposed to severe conditions.

I have used por 15 plenty and I now consider it a bodge job, a temporary fix - but one that will bite you in the ass when you go to get it off. It's 10x harder to remove than anything but epoxy paint. If prep isn't right, it will peel off in sheets.

It is nasty stuff, glumpy and thick. It dries all deep and black -I used the black stuff - but though it looks good from afar, inspection will show a lot of fish eye, regardless of prep - seems to react with certain steel?

It will never come off your hands

The worst part, though, is that it is impossible to touch up. You have to sand it or it will peel like mad, but you can't do a decent job feathering it in as it has like 'cleavage' points where the edge will start flaking and peeling like in sheets.

Once you have put it on, it is i nightmare to get it off is a big negative. I can still smell the lead-tinged dust (there is lead in it) from a week of sanding it off my bus. I did have a mask, but like I said, it was a real nightmare. I'll never use that stuff again. I'd rather use rustoleum, even.

I POR'd my fuel tank and have to say it seemed to do a great job.

But the other part of your question. My two trunks were pretty knocked around after nearly forty years of use. I wire brushed the entire area and rust treated and primed any areas that were suspect. In most places the original paint was good.

I found a metallic grey single pot paint that I rolled on. It leaves a hammer finish, looks really good and left a hard and serviceable finish. If you are interested I will put some phots up after work tonight and give you a description of the paint. Considering most of the area is covered with carpet or spare wheel etc it doesn't matter too much unless you are totally into originality.

Thanks for all of the great input. I have read on this board before about how difficult POR-15 can be to get off of the metal (if you needed to). I only want to restore this car one time. :-)

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.

All paint will fail eventually. por15 is no exception. Peeling and flaking is how that failure will manifest itself. There is still decent por15 on the bottom of my bus, but a lot of it has not held up well despite preparation bordering on the obsessive. 10 years is not that long a time to have to go through the trouble it's going to be to get it off.

The subject has been covered pretty well.....but one thing I had read about it...If you plan on top coating ( as you should) it must be done very soon after applying POR. I have heard many complain about it peeling and have seen it on a car.

I couple of years ago I consulted a NJ Porsche restoration expert about it.....He seemed reluctant to speak to my specific question but gave me two alternatives with very lengthy explanations. That told me he was not a believer.
He recommended two things
Benjamin Moore Ironclad primer ( Have it in the garage) and Rustoleum damp proof primer ( don't look for it at big box home enters) ...Both come from specialized paint stores.

I have used in interiors after stripping tar and cleaning up. Add new tar and then do Urethane color. A PITA but I like results. In engine bay it could work but have not used to date. AS you see like belly buttons everyone has an opinion.
Good luck

You know, i was just thinking about this as i vainly tried to get my por 15 to stick to non rusty metal. You may remember when eastwood used to be king, advertising some sort of glass flake technology. Now clearly they want a rust converter at the first step. Por 15 seems suspiciously like super glue for rust. The most damning thing i can say is they are now suggesting you use a fiberglass material and thickened por 15 to repair missing floorboards. Bondo was in real terms a miracle. But people took it too far. I say cut without pity, and then maybe think about por 15 . Some people say it smells like epoxy, but to me DP-40 smells like victry.

A 914 should have the trunks painted body color. I don't like the look of different color trunks than the body. Black trunks look like a bottomless pit. If you are going to paint it anyway then why not paint it body color? Unless you like the look of black then o/k, it's your car make it how you want it.

Just my opinion.

As far as por goes I welded in cage for my buddy & he used por15 on it. I think it looks horrible. He spent a ton of time prepping as per instructions but it has many deformities in the finish.

I por 15'ed my entire cabin after removing tar. I kept it out of the sun and put a carpet kit in int. Havent touched it in 10 years. Still looks like it did and I brushed it on.

I have had excellent luck with my hell hole and the exhaust heat shield which I POR 15'd about 4 years ago. No issues whatsoever. I used their Marine Clean degreaser and their Metal Ready prep as well - and I followed the directions completely.

Since I believe and have my own proof that POR-15 works well with the proper preparation, over the past 4 weeks I removed all the tar out of the interior, wire brushed it down to bare metal (or light rust), and coated the floor with POR 15. After drying for a week plus, I applied 1-2 layers of a Dynamat equivalent - Hush Mat - and a layer of Second Skin Luxury Liner Pro on the floor and the firewall.

Test drive tomorrow or Wednesday....

I would never use POR-15 on my car. There are plenty of products that actually work.

I never knew so many varying opinions on POR-15 could be had.

What are the other choices then? I'm mostly thinking about what to do with the trunks/engine bay/underside/cabin. In the end, I think I would like to have the trunks and engine bay painted as the rest of the car is. However, underneath the car and the cabin will be areas rarely ever seeing daylight.

Thanks for the feedback. It would be great to hear what you think about the Hush Mat/Second Skin combinations in your car. I'll be looking to sound proof my ride as much as as possible considering the noisy Type IV out back.

I have heard the good and the bad stories, and I have used POR-15. What I have found is that if you FOLLOW THE INSTRUCTIONS to the letter, you will get great results. There is a post above, complaining about removing the rock hard POR-15. That is how it is supposed to work. I say try it, then you will know. I basically painted my whole car with the stuff. inside the gas tank too. Here are some shots:

my trunk after a patch job. the bubbles are either the "metal ready" or the "marine clean" working. This is my 1st time I welded in 30 years, so don't be critical. I also used the POR patch as a seam sealer, also highly recommend that product.
Click to view attachment

POR patch is black.

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I ruffed it up with a light sanding then painted over the seam sealer. It will look even better when the carpet is on top of it.....

Click to view attachment

I used Por15 on the inner floor pans after stripping them and replacing the driver' side rear portion and doing the necessary prep-work. I also used it on the underside of the replaced floor pan piece and followed it up with 3m rock guard (or whatever it's called. 7 years later the it has all held up perfectly. No complaints.

If you really want to do it, strip it to metal, epoxy primer, real catalyzed paint, then undercoating. None of these things come in a rattle can, and most require specialized equipment to apply.

However a scrub down with a wire brush to remove loose material and a new application of undercoating should be sufficient. Pressure washing with solvent to get rid of oil/grease would be prudent as well.

I think the bottom line for me, is that there is no magic potion - no perfect cure. Everything has a up and a down side and the best way to prevent future rust is to limit exposure to water, and more importantly eliminate areas where water can sit trapped. It's not splashing through a puddle that rusts our cars. It's the pile of dirt behind the rocker that soaks up water and holds it against the metal. It's the loose undercoating/seamsealer that allows water to wick in behind them where that moisture will sit for a long time eating at the metal. Remove those hiding places, and limit exposure and the car won't rust appreciably for your lifetime.