[sgarnett]

The two most common uses for "weld-through" primer seem to be:

1) Protecting things that will sit around for a while before being welded, mainly in construction and manufacturing. For this purpose, consensus seems to be that it's worth the time to go ahead and clean off the primer before welding, and to paint afterward.

2) Protecting metal that will be inaccessible after the weld, such as in auto body work. That's what I'm concerned about - lap joints on the chassis where it won't be possible to completely surround and "seal" the overlap with an unbroken weld bead, or to get the paint where it needs to go afterward.

I'm also specifically concerned with mig welding (using 75/25 gas unless that's a problem).

The most common type of weld-through primer seems to be the "cold galvanize" zinc-based stuff. From what I can gather, it isn't great to weld through (but I'm not sure if that's a mig-specific statement), the fumes are toxic, and the zinc may weaken the weld. For welds to chassis sheet metal, the weld will probably still be stronger than the surrounding metal, so the weld compromise should be acceptable if it is't too difficult to get passable results. Supposedly, the resistance to saltwater corrosion is pretty good.

Aluminum-based primer such as Bloxide will supposedly produce a better weld. The aluminum apparently acts a deoxidizer/flux, and leaves a corrosion-resistant aluminum oxide coating on the bead. However, I've only seen the stuff sold for the purpose of protecting parts for a few months prior to assembly. I don't really care if the surface remains weldable without cleaning long-term, so minor surface corrosion of the aluminum itself is OK. What I don't know about is any long term protection for the underlying steel. I don't know if aluminum provide any galvanic protection, or if it's just a barrier coating.

There's also copper-based primer. It seems to be new, and I can't find much information. The only source I've located is in England.

Clearly, it's better not to use the weld-through stuff if it can be avoided, but how bad is it? How much protection does it really provide, or does is burn away near the weld to the point that it won't really provide any lasting protection? How hard is it to get decent results welding through the primer? Has anyone tried the aluminum or copper stuff? Is there a US source for the copper if it's even worth trying?

Thoughts/experiences?

This post has been edited by sgarnett: Mar 9 2006, 03:02 PM

[prockbp]

well.. when we perform test welds for destructive/non-destructive testing purposes, the coupons must be as clean as possible... anything on the surface will cause impurities in the weld

but you ask how bad is it to have impurities.. well apparently it's not that bad because only aerospace companies and nuclear power plants worry about it.. most of the welding you see/use/purchase is not up to testing standards, that's just how it works in the "real" world of welding(i'm not saying that's the right way to do things, it's just the way that welding is done)

welding through any type of zinc based coating sucks.. the zinc causes all kinds of spatter.. if i weld through zinc coatings i move slow and keep a really hot puddle going.. the heat from the hot puddle will help burn the zinc coating off of the surface before you get to it with the weld.. Flux-core wire will help raise the temperature of your puddle if you're having trouble with the solid wire

i have welded on some super thin sheet metal that had a Galv-Alum coating and it was pretty much the same story as any zinc based coating..

i would imagine that a copper based coating would be just as bad



your situation does not sound ideal for testing purposes, but it will most likely be acceptable as far as 'real' world standards go...
i'm a welder with an associates degree in welding technology and if you asked me to lay down an acceptable weld with a protective coating in the way, i would feel confident that it could be done...
you can always clean the coating off before you start welding

This post has been edited by prockbp: Mar 15 2006, 05:38 PM