Stovebolt.com Forums The Shop Area Paint and Body Shop Rust under relatively new primer?

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!

You might have pin holes in the metal where moisture can get in from the back side.

What was used for the blasting media?

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.

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

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.

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..

I love garnet!

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

SPI is the best you can get if applied properly. If it was soda blasted, that is the problem.

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.