epoxy or por15

[unimogger]

I'll make the concession that I have had one bad experience with por15 many years ago. But, I was young and probably bungled the prep, so I'm willing to give it a second chance if its the right material.
I'm a week or so away from having my bronco frame, and the underside/ engine bay of my tub blasted. Epoxy has been my go to for several years now, and I'm inclined to just stick with it. I'd do black on the frame and grey on the floors. Then seam seal the floor, and either a second coat of epoxy, or bedliner underneath. Several people have been trying to sway me towards por15; they swear by it, and seem to feel its better suited to my use.
So, to people that have used both: what do you think?
Does anyone have an understanding of the composition of por15? (I'm not really convinced that a single stage material can beat the toughness and hard cure of a good epoxy).
Any experience and opinions welcome, and thanks in advance.
SD

 

[widowmaker77]

I went back and forth for a couple months on this same thing. After reading everything I could find, I decided on sand blast then coat with kbs rust seal. Haven’t got to that point yet but I did go ahead and buy the rust seal before I changed my mind again. Lol

 

[rguest3]

Frame and Axles - Bead Blast, Metal Prep, 2 Coats Epoxy then Top Coat Black

Body - PPG Epoxy Primer - Gray on most and Red Oxide Underneath, Seam Seal, then LineX interior tub and paint the rest.

 

[chuckyb]

I would give POR-15 a second shot. It really works well if you really follow all the prep directions. Clean, freshly blasted metal is pretty ideal for POR-15....

For my rig, I went with POR-15 chassis black on axle housings, small parts such as heater, etc. Silver POR-15 radius arms, and some other parts. Black powder coat for frame. For underside, wheel wells and floor, my body man suggested tintable raptor liner, which I initially was inclined against but came out great. He tinted the raptor liner for the red oxide color underneath and then to match the factory paint on floors etc. My build thread has some pics for reference starting around pages 7 and 8.

 

[unimogger]

OK, after posting here I realized that I really should answer my own questions about the composition of POR-15. Some searching, and questions with a few guys on Practicalmachinist.com got me pretty far. Simply put, POR-15 (paint over rust)is a high solids moisture cure urethane. Chemically similar to the moisture cure foams that are used to seal foundations and shore line construction barriers. It has no actual chemical rust inhibitors, like Ospho, metal prep, or any of the phosphoric acid derived converters. It's sole means of stopping rust is that the urethane combines with any present moisture on the surface, and uses it to help cure. So, in short the urethane uses any free H2O as it cures and any rust is starved without any moisture.
Sounds good but here's the catch:
The final characteristics of the POR-15 urethane are affected by the amount of moisture present during the cure. But you never know how much rust or humidity there is per amount of paint, so what do you get? It could be under or overreacted, leading to flaking, cracking/ shrinking, or excessive brittleness. Just like the shitty finish you get for painting your car with a full moisture filter, on a humid day, but worse.
The deal killer for me, is that under the POR-15, the rust is still rust. The single component urethane will likely never match the tough semi flexible coating of a good coat of 2 part epoxy. Several of those epoxies (specifically SPI, southern polyurethane) have been shown to not fully harden. To me that sounds perfect for a frame and suspension components.
Where the POR15 does shine is where there is not a full rust removal option. Many guys that R&R machinery and implements apparently use it after an OSPHO treatment, and report that it does well. If I were not having my frame fully blasted clean, I might give it a shot.
SD

 

[EPB72]

Unimogger,u summed it really well ,I never used por 15 seen it on customers cars and really don't care for it.. I've epoxy primed plenty.ppg and more currently SPI(2 plus years)and with all the prep u describe epoxy it would be... As for spi curing haven't had a problem,,as for ospho under there primer there is concerns with adhesion and fillers applied over both,,,

 

[unimogger]

As for spi curing haven't had a problem,,as for ospho under there primer there is concerns with adhesion and fillers applied over both,,,

To clarify my post about the spi epoxy: I didn't mean that it didn't cure, I meant that it didn't cure to a brittle hardness. Some posts on the SPI actually shows that a block of cured epoxy will have a measurable durometer, in other words it is slightly flexible like a hard rubber.
As for ospho under primer, I have seen and read that people like to hose it on and just leave it. Thats asking for trouble IMO; leaving dried unreacted phosphoric acid, and expecting a decent finish over top of it. I suspect that's why people have trouble with paint or filler not sticking. My method has been to let the ospho do its thing, then neutralize everything with a baking soda solution, and scrub it down with a scotchbrite. Heat dry and epoxy prime, so far this has been a good process.

 

[bronkenn]

If you plan on using SPI products , you might want to check with Barry, the owner, about putting ospho under his epoxy. He does not recommend it.

 

[fordguy]

I do not have a preference but I have seen por15 last 20 plus years down here in the humid south. My frame ie epoxy because it was dipped like new car stuff.

 

[elan]

If you are hesitant to get involved with Por-15 on the entire frame then you might consider using it to seep into sandwiched metals (riveted/welded) where sprayed on epoxies will not go. Por is very thin and seeps well.
I do notice there are a few remarks about painting over rust. I'm curious why anyone would paint over rust.

 

[CraigS]

That's what seam sealer is for. You don't need to get between metal sheets to stop rust, you just need to stop moisture and oxygen from getting there, so a surface covering works fine as long as it's impervious. Epoxy will seal out O2 and moisture if you get a good coat. My frame is epoxy with a clearcoat on it, just because I had clearcoat sitting around. That will keep UV out of it as well.
As for ospho, no acids under epoxy are recommended. Either etching primer OR epoxy, but not both. I know ospho isn't a primer, but it is acid.

 

[hyghlndr]

Sorry, not a fan of por 15. By the time you have it cleaned and apply it, it ends up flaking off in big sections, had it on several cars. I would go with plain high quality paint or maybe KBS.

 

[bronkenn]

Been using POR15 for 25 years and it has worked very good. Follow directions and it is a good product. One of the things I like about it is it weeps into all the nooks and crannies. I don't paint it over rust as I sandblast the metal first.

 

[bronconut73]

been using por15 for 25 years and it has worked very good. Follow directions and it is a good product. One of the things i like about it is it weeps into all the nooks and crannies. I don't paint it over rust as i sandblast the metal first.

this/\/\/\

 

[elan]

Sigh...…..If the metal is properly prepared then Por-15 paint works very well. The metal must have tooth in order to be effective. This is why Por states using metal ready as it etches the metal.
In addition I clean as much of the metal I can with the Por solvent until the paper towels look clean, wipe after wipe. It takes a while to do it right. Also Por-15 is not meant for sheet metal (fenders, doors as an example) if anyone may have thought that.
Old metal is best. Now, if a frame is pristine then you will need to remove all old paint and use Metal Ready. If you are blasting, bead or sand (as an example) then simply clean with Por solvent.
I have other tips. The biggest tip is, how to preserve Por-15 in the can to last for years, not a week.

 

[bronconut73]

Sigh...…..If the metal is properly prepared then Por-15 paint works very well. The metal must have tooth in order to be effective. This is why Por states using metal ready as it etches the metal.
In addition I clean as much of the metal I can with the Por solvent until the paper towels look clean, wipe after wipe. It takes a while to do it right. Also Por-15 is not meant for sheet metal (fenders, doors as an example) if anyone may have thought that.
Old metal is best. Now, if a frame is pristine then you will need to remove all old paint and use Metal Ready. If you are blasting, bead or sand (as an example) then simply clean with Por solvent.
I have other tips. The biggest tip is, how to preserve Por-15 in the can to last for years, not a week.

I have applied all of your POR 15 tips through the years adding them to my own.

As you've said many times...
It's good stuff when used right