[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

[sgarnett]

To further complicate the mix, Wurth makes an aluminum-zinc blend called Alu-Zinc. They claim it produces cleaner welds than the pur ezinc stuff.

[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

[sgarnett]

Well, cleaning the coating off before I weld kind of defeats the purpose. I want it as intact as possible right up to the weld, because I can't get to it to replace the coating afterward. Failure of the weld itself is a more acceptable risk (in other words, a less critical failure) than corrosion of the sheet metal structure underneath.

I tried some test welds with the U-POL zinc and copper formulas. If anything, the copper was a little harder to weld through; it just didn't feel as "smooth". Copper is also the wrong polarity (relative to steel) for galvanic protection. It's simply a barrier; and once the barrier is breached it may actually promote corrosion. As far as I can tell, the only reason to use copper is it's lower toxicity.

I just received a can of the Wurth Alu-Zinc, but haven't tested it yet.

Where I live, they've become quite rabid about spraying the ice-preventing salt solution (not actually NaCl, some other salt) on the roads anytime it might get turn cool, so it has become harder to just avoid the salt (and my "beater" truck get shorrible gas mileage and is rusting badly.

[nape]

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..

My experiences are pretty similar except I've had better luck with galvanized steel then zinc plated steel. The stock I was welding was 20ga square tubing. I tried a variety of settings trying to get a better weld out on either piece since I've probably welded 2500 pieces of each variety [with 4 welds per piece] depending on availability of material.

Just a few words from experience, be careful welding anything zinc as it'll give you a wicked headache even with a fume hood if you do it long enough.

I'll agree with prock as far as "real world" welding goes, too. I can't believe some of the poor welds I've seen from temps that actually hold together and bare metal is always better.

What are you trying to weld, Sean?

[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.

[sgarnett]

What are you trying to weld, Sean?

The most pressing project is installing the UMI square tube SFCs. I will probably be tacking them to the sill, but won't be attempting a full seam weld (partly to limit heat and mostly to promote drainage). It isn't possible weld all the way around the mounting plates, so "salt" (or whatever it really is) will collect behind them.

Retiring the car to racing service only ain't gonna happen - it's more likely to be retired to daily driver service only eventually. The crap they are spraying on the roads now is far more corrosive than the rock salt ever was, and paint alone is totally worthless against it.

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.

What do you mean by "spits up"?

Since I won't be doing a full seam weld, is it better to avoid the spot welds?

This post has been edited by sgarnett: Mar 16 2006, 07:00 AM

[Blainefab]

What do you mean by "spits up"?

Since I won't be doing a full seam weld, is it better to avoid the spot welds?
[/quote]

A little blob of molten metal gets flung back at the nozzle - it happens when welding over spot welds, which can be avoided, and also sometimes when welding a seam, which can't be avoided - like the seam at the bottom of the sill that gets stitched to the SFC's. It seems like there are denser pockets of primer/galvanize trapped between the metal layers that explode as the puddle hits them. MIG is relatively tolerant of the blobs - they collect on the nozzle and don't impede the weld till the buildup gets excessive.

[prockbp]

Well, cleaning the coating off before I weld kind of defeats the purpose. I want it as intact as possible right up to the weld, because I can't get to it to replace the coating afterward.

i just mean to clean the coating off in the weld area... besides, the heat from the weld is going to destroy the protective coating within at least a 3/4 inch radius from the weld

[sgarnett]

I tried comparing the Wurth alu-zinc to U-POL zinc. I think I might have liked the U-POL better, but for some reason I was really "off" today and couldn't get a decent weld even on bare steel. I didn't learn much by trying a bead on the primers.

This post has been edited by sgarnett: Mar 19 2006, 12:56 AM