AutoBody101.com

Hi
I'm rebuilding an 80s BMW and repairing some rust spots. After I finish welding in the patches, I hit it with 1-2 coats of epoxy primer. But now after sitting a few months, I'm noticing little bubbles where the welds are. I ground off the epoxy to find rust is forming on the fresh welds. Has anyone experienced this before? Are there any measures I can take to prevent this?
Thanks
BB

How did you weld it?

Post some Pics, Lots of things can go wrong when welding even when your using all the right stuff and doing everything right.

Basically Its The Water in the Air when welding. Bead not melting into the metal correctly kinda complicated. But comes down to Impurity's in your weld bead. caused by more than a few things.
https://www.asminternational.org/docume ... CFAB86.pdf


I like to treat my welds after grinding with a Phosphoric Acid b4 Epoxy, And even Fiberglass over some stuff as well just for this reason. especially Mig Welds.
I like a lot of others am Guilty of removing My Shielding Gas too early, Post flow is critical when welding most Migs dont have a Post flow control but should.

yup, you have to treat the welds before priming, media blast is the usual way.

Did you fully weld it? or just spot welds, or stitches? Even fully welding patches, you can have tiny pinholes where water can seep in from behind. Always seal the backsides of your welding, if at all possible.

To observe your case closely you need to post pics of your welds. When you are doing welding in these kinds of situation you need to be extra careful about the level of humidity in the air. It might prove more dangerous than the danger you are already facing.