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Rust under relatively new primer?
#1385744 Tue Nov 24 2020 05:47 PM
Joined: Sep 2017
Posts: 104
D
Dongray Offline OP
Shop Shark
Looking for some advice. I was prepping to weld in a patch panel and noticed where I sanded through on a edge there was what looked like a reddish brown layer under the epoxy primer. There is also an area that appears to be showing rust coming through on the door pillar. I had my cab sand blasted and epoxy primed a couple of years ago and the person that did it had it back to me in a week. I don't know how long between blasting and priming. I realize it will need re-priming but am I going to have to strip the cab again? Thoughts?

Thank you!


1950 Chevrolet 1/2-Ton 3100
Re: Rust under relatively new primer?
Dongray #1385750 Tue Nov 24 2020 06:24 PM
Joined: Jul 2000
Posts: 1,099
G
Shop Clutterist
You might have pin holes in the metal where moisture can get in from the back side.


Gord
----
1954 1/2 ton 235 4 speed
Re: Rust under relatively new primer?
Dongray #1385842 Wed Nov 25 2020 01:35 PM
Joined: Jul 2011
Posts: 703
M
Shop Shark
Originally Posted by Dongray
Looking for some advice. I was prepping to weld in a patch panel and noticed where I sanded through on a edge there was what looked like a reddish brown layer under the epoxy primer. There is also an area that appears to be showing rust coming through on the door pillar. I had my cab sand blasted and epoxy primed a couple of years ago and the person that did it had it back to me in a week. I don't know how long between blasting and priming. I realize it will need re-priming but am I going to have to strip the cab again? Thoughts?

Thank you!




What was used for the blasting media?

Last edited by MPandC; Fri Nov 27 2020 04:31 PM.
Re: Rust under relatively new primer?
Dongray #1386889 Thu Dec 03 2020 12:25 AM
Joined: Jan 2019
Posts: 25
6
Wrench Fetcher
I'd ask him what he used. It's hard to tell without photos, but he may have etched it prior to priming. If it's rusting under the primer, inevitably you'll need to strip it.


1949 GMC 100 1/2 ton
1969 Stingray convertible
2011 RS/SS Pace car
Re: Rust under relatively new primer?
Dongray #1386901 Thu Dec 03 2020 02:08 AM
Joined: May 2005
Posts: 7,638
B
General Purpose
An actual good epoxy primer should not do that. It seals. Something fishy here. BE SURE it is rust. BE SURE it was a brand name two part epoxy primer on bare metal. Any good sandblaster knows it needs primed pretty quick. Except where I live smile

As Robert is asking: Some people refer to all blasting as "sandblasting". Find out what was used: sand, glass, black magic, garnet, soda, walnut................etc.

If it was a redish brown "layer", it is some sort of primer.


Watch out for careful drivers!!!
I'm away on an ego trip. Will be back on Feb 30.
I'm not an Auto Mechanic, but I play one on TV.
I charge $0.02 for every opinion and I take Paypal.
Plan B is always better than plan A, by definition.
You can't teach a new dog old tricks.


Re: Rust under relatively new primer?
Dongray #1386959 Thu Dec 03 2020 05:30 PM
Joined: Jul 2011
Posts: 703
M
Shop Shark
I've seen a few other instances where someone sanded through their epoxy primer coat while blocking and found rust UNDER the epoxy where it had been previously blasted clean. In all cases the media that had been used was sand. Most sands contain caustic salts that, when used for a blasting media, are being pounded into your sheet metal. It may be about the cheapest media out there, but it is not ideal.. I think you'll find the instant money savings results in more effort to clean that salt residue. I prefer an inert media, such as Garnet or Coal slag. They have less chance of these adverse effects, and are well worth the extra money. Especially now that the paint substrate adhesion is questionable..

Re: Rust under relatively new primer?
Dongray #1386974 Thu Dec 03 2020 07:28 PM
Joined: Oct 2005
Posts: 3,092
"Hey! I sound like Darth Vader!!
I love garnet!

Re: Rust under relatively new primer?
Dongray #1387010 Fri Dec 04 2020 03:13 AM
Joined: Sep 2017
Posts: 104
D
Dongray Offline OP
Shop Shark
Im not certain what was used I would assume sand but that would be a guess. In regards to the primer it is a 2K epoxy primer from SPI. I have done some spot checking in various other areas of the cab and have not noted rust under the primer. I might go back to the location and see what the extent is. That being said if I moved on in the build I would always be nervous that rust would rear its ugly head eventually. I new I should have kept it with the old patine and let well enough alone.

Thanks,
Eric


1950 Chevrolet 1/2-Ton 3100
Re: Rust under relatively new primer?
Dongray #1387026 Fri Dec 04 2020 06:05 AM
Joined: May 2005
Posts: 7,638
B
General Purpose
SPI is the best you can get if applied properly. If it was soda blasted, that is the problem.


Watch out for careful drivers!!!
I'm away on an ego trip. Will be back on Feb 30.
I'm not an Auto Mechanic, but I play one on TV.
I charge $0.02 for every opinion and I take Paypal.
Plan B is always better than plan A, by definition.
You can't teach a new dog old tricks.


Re: Rust under relatively new primer?
Dongray #1387031 Fri Dec 04 2020 07:34 AM
Joined: Jul 2011
Posts: 703
M
Shop Shark
Originally Posted by Dongray
Im not certain what was used I would assume sand but that would be a guess. In regards to the primer it is a 2K epoxy primer from SPI. I have done some spot checking in various other areas of the cab and have not noted rust under the primer. I might go back to the location and see what the extent is. That being said if I moved on in the build I would always be nervous that rust would rear its ugly head eventually. I new I should have kept it with the old patine and let well enough alone.

Thanks,
Eric


Eric, not to split hairs here, but is it Epoxy or is it 2K? They are not one in the same. About 3 coats of epoxy and it will seal out moisture from your sheet metal. 2K is their high build primer, which is a more porous paint, it does not seal. So if you indeed used 2K primer, it is possible that moisture crept through the primer and rusted the panel. If it is Epoxy Primer, I would lean more toward media blasting/prep being the culprit.

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