So, I have read that using a weld through primer is a must in a lapped joint. I'm mig welding in cowl patches in the 39 GMC cab and since it's a two piece patch, I'm wanting to pre-tack the inner cowl to the outer cowl, and then fit the patch to the cab... My understanding is that the primer is supposed to be applied between the joints, and then plug welded in place. Well, it seems that the primer causes a cold weld... Completely cold. Like zero penetration. I may as well have been trying to weld steel to copper...Because I effectively just patched my drilled holes meant for the plug welds... What is the proper use of this stuff?
I have used a couple different brands in the past. Never had a problem with it. I always turn the heat up a bit more than normal, and ALWAYS make sure you let it dry completely before trying to weld.
I also tryed that primer and the only way I could get it to work is scratch the paint where you are trying to weld the I was not impressed Posted using the Full Custom H.A.M.B. App!
Posted using the Full Custom H.A.M.B. App! The idea behind the weld through primer is to seal the lapped bare metal. A helpful tip in the future is to use a spotweld bit or a carbide burr to scratch out the primer within the hole. Most weld thru primers suck as far as conductivity goes. I use Shield brand made by Sherwin Williams and can be bought st NAPA. It looks useless when applied but it actually conducts better than bare metal.
I use U-Pol stuff at work all the time and have zero issues with it when using it with "plug" welds . Works like a charm , even without grinding the primer out of the hole . Using a weld-through primer with the spot welder (Lenco-Spot) is a bit more touchy though . It's gotta be set just right & everything clean and tight . I've done a lot of resto work , BTW (full time , 20 yrs.)
I use epoxy on both panels and just clean the epoxy out of the holes... Here's the epoxy applied to the adjacent sides, I use a letter A size (.234) drill bit for my plug welds, and to clean the epoxy from inside the hole, I use a letter A that has been flattened and backfaced to form a cutter similar to an end mill. It cleans the paint well, and hardly touches the metal on the adjacent panel..
Just about all the major brands clogged in the nozzle after a few uses no matter how many minutes the can was shook or turned upside down and cleared after use. We found that Tractor Supply sells cold galvinizing spray that works great and is a lot less than brand names. Next up would be UPOL copper colored stuff. As stated above use a dull drill bit to reclean prepunched rosette holes. Turn the heat up a bit and stay longer before you move the puddle around. They do have a tendency to cause splatter so cover the tender bits!
damn MP&C got there first. Since we blast everything in house and cover in epoxy we barely ever use weld through any more, but there are times when needed.
I do it like MP&C. A few years back I used some in a spay can,I think it was 3m brand I could not get it to weld clean or get good penetration. It was on a customers truck and he insisted that I use it. This was a lap joint on a tranny tunnel both sides needed to be welded on both sides. It looked so bad I cut it out and broke it apart. The weld seemed to be on top of the paint. A lot of the paint in the seam burned up and was not bonded to the metal most wiped off with my hand. When I ground the welds they were contaminated. Never again! No matter how you do it you don't breath those burning paint fumes.
Best stuff I've tried. It's required by some OEM for modern repairs. It has some uses in restoration. It melts and wickes back towards the weld. Less harmful fumes when heated. The most important thing is to seal the joint with quality seam sealer. No oxygen no moisture = no rust. It's worth stepping up to the two part stuff. Some good urethane seam sealers being made too.
Never had issues using it, being zinc, aluminum or copper type weld thru primer, just don't lay too much of it, apply it like wash primer (can weld thru that too) crank the heat up and not too much wire speed either. I never grind it off before welding. Been doing it for many years. There even used to be a real good weld thru glue to use on some new cars but that was real nasty to smell coming out the cartridge and worse while welding, but once dry and welded their was no way moisture could get in and it was strong as hell.
We had a rep from 3M come to our shop.We were bitching about this problem.He said it should not be called weld through primer.It is misleading calling it that.he said Yes use it but before you weld,don't put it on with heavy coats,just enough to seal the surface. clean the spot to the bare metal that your welding.It can take the heat ,not like paint that burns away. I have had the same problem but since doing it this way have had better welds.
YUP Buddy is right. Up here a can costs 50.00 but it is the best i found. Like it's not even there when welding. Has the least of burn away from between the layers to. 3m was the worst of the bunch. I used to take a broken drill bit and clean out the hole first but not with U-Poll.
Thanks SO MUCH! You guys have been a tremendous help. Hopefully I'll be able to head out to the shop to test this out. THANKYOU THANKYOU ALL!!
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