[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

[Blainefab]

I have not found a situation on a car where I felt I needed to use the stuff, but agree that zinc is bad stuff to vaporize. I use a chemical stripper to remove paint and primer, then take off the galvanize with a medium 3M woven abrasive. That does a good job on the pan without unecessarily removing steel from the thin sheet metal. The subframes seem to be plated thicker, and the metal is thicker, so I use an AlOx abrasive there. Seam welding, especially on the sill, is a bitch. I grind the edges down to bare steel on edges and 1/4" up the sides, but there is plating trapped between the spot welded layers that spits up. I got the MIG welder a couple years ago when I realized how hopeless it was to TIG on that stuff. With MIG I use a little less V, continuous bead, and when it spits up just pause for a moment to fill the crater. About every 6" or so I stop to clean the nozzle and take a breath. I do SFC's with the car on wheel stands, laying on the floor. The fumes rise, so when I finish a section I crawl out of the garage and let it air out. My new shop has 12 whirlybirds on the roof so should clear fast.