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Old 07-13-2008, 08:44 AM
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rust comming thru epoxy primer

I sandbalsted the cab of my 50 chevy and primed it with SPI epoxy, my goal was to get all the rust off and have a good base to start. now after 6 months of sitting inside my shop I looked at it and rust is showing in a few areas. the truck was blasted to bare metal then sanded before priming. I was under the impression the epoxy would completely seal off the metal. what now?

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Old 07-13-2008, 08:51 AM
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Even sandblasting can leave behind minute flecks of rust. Sand blasted metal should be first treated with a rust converter before being coated with epoxy. I am certain there is nothing wrong with the SPI epoxy. It's obvious that the rust was not completely eliminated. What about the back side of the surface?

Vince
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Old 07-13-2008, 08:58 AM
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I am not sure as to what happened here..anyway I copy and pasted the question to SPI tech support so you can get a direct answer..My thought is that the epoxy could have been shot a bit thin in those spots but since I have not had that issue and no one else has had that issue that I know of...it is a head scratcher..

Sam
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Old 07-13-2008, 09:05 AM
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Quote:
Originally Posted by 302 Z28
Even sandblasting can leave behind minute flecks of rust. Sand blasted metal should be first treated with a rust converter before being coated with epoxy. I am certain there is nothing wrong with the SPI epoxy. It's obvious that the rust was not completely eliminated. What about the back side of the surface?

Vince
unless I am wrong, SPI advises not to put anything on the metal before the epoxy. even if there were minute rust particles the epoxy is supposed to seal the surface. I went with the SPI because it is supposed to be the best. the rust showing looks like flash rust comming thru. I will try to get pics. we have high humidity 85-90% but this cab has been stored inside my shop since primed, looks like the only solution would be to get all the primer back off, that would suck
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Old 07-13-2008, 09:06 AM
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Rust is rust nothing converts it. That stuff in a can that ya brush on and it turns it black supposedly converts it. Blah waste of cash. Now when you sandblasted and then sanded it was there any black specks????? Because rust will look black when ya blast or sand it but not get all of it off.... It's not gone until it looks like metal. And even then there is microscopic traces of it left embeded into the metal. On another note anywhere you touch it with your skin or drip sweat on it. Will make it rust there...
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Old 07-13-2008, 09:07 AM
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Quote:
Originally Posted by OneMoreTime
I am not sure as to what happened here..anyway I copy and pasted the question to SPI tech support so you can get a direct answer..My thought is that the epoxy could have been shot a bit thin in those spots but since I have not had that issue and no one else has had that issue that I know of...it is a head scratcher..

Sam
thanks, I put 2 good coats on it
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Old 07-13-2008, 09:09 AM
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[QUOTE=HalfOunce]Rust is rust nothing converts it. That stuff in a can that ya brush on and it turns it black supposedly converts it. Blah waste of cash. QUOTE]

Total disregard of coatings technology IMO. You do not have to perform a very in depth internet search to confirm.

Vince
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Old 07-13-2008, 09:10 AM
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Quote:
Originally Posted by HalfOunce
Rust is rust nothing converts it. That stuff in a can that ya brush on and it turns it black supposedly converts it. Blah waste of cash. Now when you sandblasted and then sanded it was there any black specks????? Because rust will look black when ya blast or sand it but not get all of it off.... It's not gone until it looks like metal. And even then there is microscopic traces of it left embeded into the metal. On another note anywhere you touch it with your skin or drip sweat on it. Will make it rust there...
no black specs it was bare metal, microscopic I would not be able to see. it was wiped down with wax and grease remover before priming. no touching or sweating on it
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Old 07-13-2008, 09:38 AM
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I don't put anything on bare metal before epoxy primer. No metal prep or rust convertor at all. I do clean with W&G remover, but that is all. As far as I am concerned, adding any other coating is just something else to prevent the epoxy from adhearing properly.

Vince.... I always respect your opinion, but in this case, I have to disagree. Sure, there is a lot of information on the internet pushing those rust cures, but there is also a lot on other subjects, that are wrong, like magnets that increase fuel mileage.

Aaron
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Old 07-13-2008, 09:39 AM
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Quote:
Originally Posted by 302 Z28
Sand blasted metal should be first treated with a rust converter before being coated with epoxy. Vince

I don't think sandblasted metal should ever be treated with any kind of acid prep or "converter", in fact I don't think you are supposed to use anything on on the surface after sanding except maybe wax&gease remover before applying epoxy. I have mentioned several times a little unscientific test did I nearly three years ago using epoxy on rusty panels. This "test" was never meant to prove or disprove ANY product and was simply to see for myself how rust would fair under a coat of epoxy. The panels had surface rust with pitting just starting so I sanded them down and no special care was taken to remove all the rust, the specks remained in the tiny pits along with some very light rust intentionally left on one end of that particular panel. Basically I tried to simulate what someone might do if they used a rust "converter" and just left the rust instead of properly removing it. I did another panel in POR 15 and these have been outside nearly three years now. The panel with the POR 15 looks just like it did the day I painted it (it has a light coat of paint for UV protection) with the only rust starting around the edges where I probably did not get good coverage. The thing is the epoxy has held up JUST AS GOOD without even the rust forming around the edges, I have removed the epoxy in some areas, even where the light surface rust was left, and there appears to be no further rusting. I am not saying epoxy is a cure for rust and it would NOT be OK to use as a rust "converter" on rusty parts but it sure seems to me that small pits or tiny specks left (or newly formed) on a sandblasted panel should not cause a problem such as we have here. This test was done with a cheap epoxy, Omni MP 170, and I am sure the SPI is much better quality in every respect so my thinking would be some kind of acid contamination, possibly during the blasting it's self, or the epoxy was too thin in those spots which does not seem to be the case. I would suspect some kind of contamination during the sandblasting process but this is just my opinion of a couple of possible causes.
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Old 07-13-2008, 10:05 AM
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I blasted it with black diamond abrasive. I did not use sand, what could have contaminated it?
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Old 07-13-2008, 10:16 AM
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Even pretty severe rust pitting would be "buried" under the epoxy and sealed off and NOT rust anymore. So if it was cleaned well, with no rust showing, I seriously doubt it has anything to do with some microscopic rust that was primed over. That is giving a free pass to the primer AND the application.

Most any 2K primer is going to seal off the metal well enough to keep it rust free IF (and this is a BIG "IF") it was TOTALLY covered with enough film thickness to do it's job.

I have seen many times where someone "covers" something with primer and because it looks all one color they think it is protected. But some thin films can allow moisture thru.

Do you have photos?

And, just where is the rust coming thru? Is it at edges that could have been applied thin? Or is it in the middle of the panels where it is likely to have been applied properly?

Brian
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Old 07-13-2008, 10:16 AM
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What grit or grade of blackdiamond, when blasting the blackdiamond will profile the metal leaving a rough surface. Could be that the primer coat was not thick enough to cover properly nad started rusting through the very thin primer.
The blackdiamond works very well at removing coatings and rust from any base metal, great stuff. TF
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Old 07-13-2008, 10:39 AM
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Is this rust appearing at joints or seams? If rust is trapped in an overlapping seam it will remain even after sandblasting and then to compound the problem it will be all but impossible to completely seal it off from the atmosphere and moisture.
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Old 07-13-2008, 10:45 AM
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[QUOTE=302 Z28]
Quote:
Originally Posted by HalfOunce
Rust is rust nothing converts it. That stuff in a can that ya brush on and it turns it black supposedly converts it. Blah waste of cash. QUOTE]

Total disregard of coatings technology IMO. You do not have to perform a very in depth internet search to confirm.

Vince
No need to do an internet search when i have tried it long long ago and they simply cover it thee old "out of sight out of mind" does not fix rust period.
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