Ospho bare or rusty metal, seam sealer, THEN primer?

Seam sealers or caulking always over primer. Never on bare metal.

Found out the hard way on that one. I welded in a headlight bucket, used ospho and then seam sealed. I did some welding on the back side and it caught the seam sealer on fire. I scraped the burnt up sealer off and to my surprise there was already rust underneath it. I always prime first now. Thankfully that was the only area I did this to.

Why was there rust if the area had been Ospho'd? Did the seam sealer counteract its effect?

I'm no chemical engineer. My only conclusion was that there was some chemical reaction type deal going on. This was not an instant reaction. I seam sealed it weeks after the Ospho. Primed it. Seam sealer was on there a week. I welded near it. The seam sealer caught fire. It sat around a week before I scraped it off and found rust. The burning seam sealer might have even given off some crazy chemical that neutralized the Ospho and ate into the metal.

All I know is that I prime first now because it's better safe that sorry.

Ospho doesn't kill the rust, it neutralizes the surface of the rust. If it's surface rust, it's a good solution. If it's a thick rust bubble, treating it with Ospho is like pissing into the wind. It's gonna come back.

Wire brush thick rust out, Ospho to treat the remaining surface rust, then I like self etching, 2 part catalyzed primer. The paint is made to etch into metal (like Ospho), so any Ospho residue won't screw the paint. Then seam seal, let dry, and paint again.

Is your seam sealer porous??? Once it caught fire and the paint layer went away, maybe the sealer acted like a sponge.

If it's going to be months before I have an area large enough to justify getting a gun ready to prime, is the area ok and stable having just been treated with Ospho?

And what does welding in the vicinity of or on the back side of an Ospho-treated panel do? Can I presume it'll need re-treated?

Can filler be applied directly over Ospho?

I had bare metal doors sitting in my garage for a year with just Ospho with no rust issues. I had bare metal flares with Ospho on my car for the entire winter with no issues. Still bare and shiney today.

When I weld I treat the area again. Don't know if heat affects it. Wiping it again seems like quick and easy insurance.

Don't know what the deal was with the seam sealer. Don't think it was porous. Stunk like hell when it burned. Learned not to seal any area I was going to weld near.

Treat the metal, prime it, then seam seal.