Seam Rust

[Foot Stomper]

When two panels are being welded together, it's imperative to use zinc or copper weld thru primer. These products will prevent rust better than anything else and are the easiest to weld. Most folks will put too much on, won't wait for it to dry or buy the cheapest product and use it once or twice and then refuse to use it again, then go back to welding bare steel because it welds far easier than steel coated with zinc or copper. These products will help to keep your spot welds smaller too but the main thing is they don't burn off leaving a large area of unprotected metal between the panels.

If you're butt welding, this product has zero purpose. It's only meant for welded overlap.

[hotrod1]

Here are a couple of pics.

An overlapping seam was taken apart to find rust. You are right that it needs oxygen to really start rusting but I have seen rust creeping out of these overlaps causing rust discoloration of paint. I would like to stop this from occurring. Also see a cut apart overlap to see what is really happening.

[BlueJeep]

That's what made my project take way longer than I wanted it to.....

On mine I made a judgement call. If the rust was swelling the seam I cut it apart and cleaned it and put it back together. Some of the seams I used weld through primer, some I used POR 15 and then ground clean where the spot welds went, and some I left bare and relied on my seam sealing and epoxy primer to take care of. In my opinion, you have to decide how deep you want to go and stop there.