Consensus on Lap Joint Weld Prep? Options

[bkrantz]

So based on experience and expert knowledge, what's the best way to prep metal for lap joints, especially with plug welds? No paint or treatment? Ospho? Weld-through primer? And what about seam sealant after welding before paint?

[doug_b_928]

I've been following a similar procedure to most here. But, I've been using POR Metal Prep on the old metal after media blasting. I contacted POR and asked them for their recommendation re wax and grease remover prior to epoxy. Their response was that wax and grease remover might adversely affect the paint adhering to the metal. They said the epoxy (or POR paint) should be applied within 48 hours of the application of the metal prep (once dried, of course), and if outside that window to reapply the metal prep. Then apply the epoxy primer (POR paint likes to adhere to any flash rust that forms but they said for epoxy primer to either leave it or possibly sand off any flash rust, they weren't sure which would be best in the case of epoxy primer). So, I've been reapplying my metal prep (and neutralizing with water, of course) the day before epoxy priming (so the metal is completely dry) and then just blowing off the area to remove any loose, chalky zinc coating. It seems to be working very well. This is for structural repair areas and not on the cosmetic parts of the car. YMMV.

Regarding weld thru, I started out using zinc and using a broken/flat drill bit to clean it from the plug weld location, but, despite doing my best to remove it from the plug area, I didn't like how it welded at all. I switched to UPol copper and I don't even remove it from the plug weld location. It welds thru great IMHO. I only apply it to the projected 'heat affected zone' of the weld area. Apparently the theory with zinc weld thru is that it's proximity to the weld (where it was burned away during welding) is enough to inhibit rust even in the burned area. I'm hoping the copper works the same way.