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That looks like flash rust from being exposed too long? Knock that stuff off again, clean it with acetone or lacquer thinner and prime with epoxy primer. You can buy epoxy in a rattle can. I think that’s the best protection you can get. Self etch is a can of worms because it does not protect from moisture and has several compatibility issues.
 

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thank you for all the info. First timer here doing any of this. Do I need to seal the seams prior to priming? Is there somewhere I can go to get a picture or diagram of all the spots that need to be sealed. I still need to work on the floorpan that was replaced so I'm only to go down to the weld line right now.
this Is where the compatibility issues come into play. Some seam sealers are ok on bare metal, some are not. Most are ok on top of epoxy primer. Most are not ok on self etch primer. 3M makes good quality seam sealers and I’ve had great luck with them. They are a bit expensive. The product data sheets are helpful for determining which substrate is acceptable. I think the 2 component (2k) sealers, are ok on clean, bare metal. They require a special applicator gun. The 1k urethane based sealers are not recommended on bare metal or self etch primer, to my knowledge. It gets to be a confusing puzzle fast which is why epoxy primer, even though it’s more work/$ to achieve, is at least easy once it’s down. You can put any seam sealer you want on it followed by any top coat you want. Like @Nova Thug mentioned, the phosphoric acid treatment is really good. You can leave the metal bare for a very long time with no rust. I used that stuff on my roof panel and left it for months (in my garage) with zero rust.

these are just my thoughts based on my experiences. Im definitely not a pro.
 

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Here is the "epoxy" 2-component seam sealer. https://www.3m.com/3M/en_US/p/d/b40066526/ The urethane single component stuff :https://www.3m.com/3M/en_US/p/dc/v000232289/. I have read that the "fast & firm" stuff is good but I've never used it. https://www.3m.com/3M/en_US/p/d/b40069454/. I have used the Barrier Bond brush on stuff inside my quarter panel repair seams with decent results. This stuff is a little more difficult to work. https://tcpglobal.com/collections/brand-barrier-bond/products/brb-ss101-qt.

Eastwood and Amazon have rattle can 2k epoxy primer. Quite expensive at $30/can but good stuff. Just be sure to plan your work well to maximize usage/minimize waste. Once you activate a can, its done once the clock expires. I think you get a couple of hours pot life. https://www.eastwood.com/eastwood-s...ickedid=602838622301&wickedsource=google&wv=4, https://www.amazon.com/SprayMax-Act...t=&hvlocphy=1025649&hvtargid=pla-569092577986

All that said, you can use self etch primer followed by a top coat of regular primer or even paint, such as rustoleum, then seam seal and top coat again. You can achieve excellent results this way, just be aware that it's not as durable as the automotive, catalyzed stuff. I used a lot of rustoleum paint and self etch primer on my project. I applied it all with an HVLP gun plus mixed it with hardener from tractor supply. The insides of my quarters and interior metal were all done that way.

Best of luck with your project. Hope at least some of this is helpful.
 
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