Undercoating: Epoxy Primer or POR-15?

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Pierps

Pierps

10 Year
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Mar 14, 2011
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I've done a lot of reading and searching on undercoating.

There are many opinions about what to use and what not to use. My question is, for an underbody that is stripped down to bare metal (with no rust), is there any benefit to using POR-15 instead of epoxy primer?

Either way, I plan to spray raptor liner on top to provide sound-deadening and extra protection against chips from rocks/etc.

From what I understand, POR 15 involves quite a lot of prep work and its only real benefit is its ability to adhere to and "convert" light surface rust. I've also read that if you're doing a bedliner-like material over the top of POR 15, you need to do so while the POR is still tacky, or else you'll need to spend a bunch time scuffing up the surface... in which case, I'd be nervous that I'd miss spots. Logistically, I can see having to spray bedliner while the POR is still tacky being pretty tough to do without the help of a few friends.

I think the fact pattern supports the use of epoxy primer, but is there some other benefit of POR 15 that I'm not aware of?

Thanks!!

Peter
 
Hi Pete,its Pete here, he,he..
Epoxy primer for sure is best...Por will not adhere to bare metal.
I had a friend do it on his 40 the 15 way and had to strip it all down but could do it with a putty knife.

You CAN NOT beat a good epoxy primer, its the s***...
I like to you a good thick coat of gravel guard after then paint, :cheers:
 
I think the trick to the POR15 is you have to follow the procedure. Using the metal ready that they also sell. I used the por15 on my cruiser and have had no problems.
 
My experience with POR-15 is that it has to have something to grip to. It will not stick to smooth, unscratched metal. If there is something to give it some "tooth", it will stick. On slightly rusted metal, POR would be better than epoxy. On really clean metal, I'd take the epoxy.
 
I would use a good epoxy primer like a PPG product (DP-90). The more I use epoxy the more I like it.
Your going to find there is a time window to top coat with epoxy too. I know the PPG has a 7 day window, so plan accordingly.

I primed my 88 with Epoxy and it's been great. No rust even though they salt the roads here like a SOB.
 
DougAustinTx said:
My experience with POR-15 is that it has to have something to grip to. It will not stick to smooth, unscratched metal. If there is something to give it some "tooth", it will stick. On slightly rusted metal, POR would be better than epoxy. On really clean metal, I'd take the epoxy.
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I use POR 15 for many projects. The metal does need to be totally clean but, before applying POR 15, the metal needs to be treated with an acid bath/solution. This process will break the surface down. I currently use the Chemfil DX579 that is available from a lot of automotive paint supply stores. Dilute it in a regular old spray bottle and spritz it on, let it etch the metal for 20 minutes and then hose it off.

It is not necessary to remove all of the rust. Heavy stuff yes, but surface rust will be scrubbed by the acid solution and POR 15 will bite through the rest.

POR 15 is an excellent primer if painted over within 24 hours. POR 15 will never chip or peal. Properly applied with a brush or roller, it will dry to a smooth finish. If it will be exposed to the sun it should be top coated with something that is UV stable such as Rust Oleum or what ever you like best.
 
I am not a fan of POR. I think it is overpriced and hard to use. There are lots of others on the market that work well, rust bullet, zero rust, etc. No one paint really is the fix it all. Phosphoric acid etch to kill the rust. Then tri sodium phosphate to clean it. Then paint with your choice of paint.
 
I usedpor and it worked great. Tough stuff. They sell a metal prep primer for the por produxts if you putting it on bare metal.
 
FOr clean metal, and body work, I would use epoxy primer.
I use POR on old things that I've de-rusted (axles, drive shafts etc). The prep is really a pain in the a$$, and if you don't prep it won't adhere. It is also a pain to top coat. Curious about the above post saying can topcoat within 24 hours. Typically you have to sand it, but you can use the POR Tie-Coat primer over the POR and then you can use normal top coats and treat it like a normal primer.

I have a new galvanized tub, and when I paint that I will use Epoxy primer. I just painted my drive shafts with POR. Both good products, but different indications....
 
Epoxy primer if it is truly rust free, pro 15 has problem unless the direction are followed 100% and even then people have had problems, it is like the other moisture cured so it pulls the moisture out of the rust stopping it, but if no rust why risk the problem, epoxy primer, chemical cured would be much better on truly rust free metal.
 
Thanks everyone for the responses... It seems there are some who love POR 15 and others who have mixed feelings. I haven't heard a single thing against epoxy primer for clean, bare metal applications... So epoxy primer it is!
 
cjgoode said:
Epoxy primer if it is truly rust free, pro 15 has problem unless the direction are followed 100% and even then people have had problems, it is like the other moisture cured so it pulls the moisture out of the rust stopping it, but if no rust why risk the problem, epoxy primer, chemical cured would be much better on truly rust free metal.
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Chris, I can't believe you didn't mention Master Series Silver.

POR must perform well in some applications or (you would think) people would stop selling it. The only time I used it was on a very rusty Vespa back when I was in college and it was an expensive, time-consuming failure. When I told the guys at my local Napa about the failure, the one person who had actually tried it said that it had failed on him too but that he figured it was his fault.

I've been hearing that the urethane primers with aluminum solids (Master Coat, Rust Bullet, etc.) are the top performers. I'm using Master Coat Silver as a base coat. It is much more user-friendly, it can be applied to new metal that is scuffed/etched and it's compatible with any primer or topcoat.

So, my undercoat will be Master Coat Silver, followed by epoxy primer, followed by Raptor. :)
 
My 1962 fj40 has Master Series Silver on every last part except the fenders, need to weld them first. Where I spilled some Master Series Silver on the wood I work on it is super solid, stuff I painted a year ago looks like the day I put that stuff on it. An area I had put this stuff on after welding and wire wheeling the metal was close to permanent. Did not clean or anything, just wire and rolled it on. Even where we put it right over the old paint was difficult to get back down to metal, where the old factor paint with that wheel came off like nothing.

There duffontap there I mentioned it, Under the hood I coated it and have now sprayed that with epoxy primer, but have not looked at it since, it has been raining hear for the last week non stop until today. Yesterday 4 more quarts of it showed up at my door so I can finish the fenders and use it on my next project. So far I am sold.

The eastwood rust encapulator I used on the jeep is failing in some spots, but then that sits in salt spray, is driven on the beach and then left with salty sand inside and everywhere underneath all the time. So maybe it is actually holding well consider the abuse. But the Master Series stuff is clearly far superior when it comes to trying to get back to the metal where something would need to get to start rusting it. Add to that the instructions actually say when applied over marginally prepared surfaces, how can you not follow those instructions exactly, everything I do is marginal at best.
 
cjgoode said:
My 1962 fj40 has Master Series Silver on every last part except the fenders. . . . There duffontap there I mentioned it
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Your posts on MS Silver sold me on it a long time ago and I'm loving it so far. Add to to all its other charms that it has very high heat tolerance which has been nice with new panel work because the damage around welds is so minimal. Also, those Master Series guys are incredibly nice to work with--superior customer service.
 
No Brainer. NOTHING will beat epoxy primer on clean, bare steel.


POR-15 isn't any more of a rust preventer than any other paint on the market. What makes it useful is that it can (and should) be applied directly over rust.
 
Yeah, that's what I suspected.

The struggle is that I will have my tub media blasted... being a weekend warrior and living in Seattle where it's somewhat humid (although not as bad as some places), it will be logistically tough to get the tub home, do body work, prep, and prime before flash rust sets in.

I'm going to try and get as much of the patching and hammer/dolley work done before I send it away for blasting. I guess I'll have no choice but to epoxy prime the whole thing as soon as I get it home, grind primer away in areas where I need to do body work, and re-prime once complete.

I've read that epoxy primer is not meant to be sanded, and if you do use filler over the primer, you need to do it within a certain open window (7 days or something like that) or you'll risk de-laminating... or else you're going to have to scuff it up, which will be tough if it doesn't sand well...

Anyway, I'm going off topic on my own thread. But if anybody has advice about the order of operations here, I'd appreciate it. I've never done anything like this before, so maybe I'm over-estimating how quickly the tub will flash rust.
 
por 15, rust bullet, master series the the other rust paints are not like any other paint, normal paint cures by the chemicals in the paint evaporating out, leaving rust under them able to continue slower with their existing moisture in the metal and rust, just like under powder coating if metal is not clean and 100% dry, but these rust paints cure by pulling moisture into them to cure, so the metal side pulls the moisture our of the rust and metal 100% stopping the rust in theory. No moisture rust can no longer be active, and the non metal surface pulls moisture out of the air to cure. So if real humid they cure a lot faster then if real real dry, where spray paint would be the opposite.

They are also ridiculously stronger and more water proof then spay paint, people who have used por15 and these other know how hard they dry, and once totally cured you have to sand them before putting any thing else on or the next layer will not stick to it, por 15 probably the worst for that. Let that stuff 100% cure and it is like graffiti proof, but the sun will destroy it, ANOTHER GOOD THING ABOUT MASTER SERIES it is UV stable. By the way I found out about the master series on this site.
 
Pierps based on your last comment I would consider using master series or rust bullet or similar. They stick to clean metal and protect it. I rolled mine with a sponge roller once my son finished stripping the paint, cleaned it to remove oil and contaminates, and rolled it on. Then I sand an area with 220 to scuff it up not remove it, and put the body putty right on top of it. Then roll those spots again when I think the body work is good. That way metal is protected even under the putty, no flash rust to worry about. I think you can also putty over epoxy primer if scuffed as well, but make sure the metal is 100% dry and clean and no flash rust at all before putting the epoxy on
 
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