Here's my latest stupid question. First, let me say that I've been using Upol's Copper Weld-thru primer and really like its weldability. But I confess that I'm becoming skeptical that weld-thru actually does anything other than part you with your hard-earned money. Here's a pic of the backs of my floor pan plug welds I just finished, showing the neat little circles where the primer burns off.

Click to view attachment

That seam is not protected from rust. Ultimately, protection is going to depend on seam sealer, primer, paint, and whatever coating I can get to seep into that seam (like cavity wax, internal frame coating, thinned epoxy, etc.) AFTER the welding is done.

I like this guy on Youtube who puts this stuff to the test. They are a little long, but interesting.


https://www.youtube.com/watch?v=dHSEpq-Hitg



https://www.youtube.com/watch?v=SK2CSJRp5js


I was planning on soaking all my seams with either thinned epoxy in a pump sprayer, or Eastwood Internal Frame coating anyway; and followed with 3M cavity wax which I'm a big fan of. I just used up the last of my Upol primer. I have a couple cans of Eastwood self-etching weld-thru on the shelf, but I hate that stuff.

I'm strongly leaning toward saving my money and finishing the little bit of remaining plug welding on my project without weld-thru, but following with the other treatments. Interested to hear opinions on this.

Lots of the pro guys just use Epoxy Primer on everything. You then take a flat drill bit and clean your plug weld hole. just a tad bit of epoxy burns off, rest stays behind. And when it is apart you can paint everything.

I have a bud that restores Sunbeam Alpines and Tigers. When he is done, he pours what ever epoxy he has left over and pours it into every opening he can find. And then spins the rotisserie for a few minutes, and watches the epoxy run out. When it stops running out of the holes he stops turning. He figures it is better than nothing.

My experience with weld through primer so far is that it contaminates the weld puddle and makes a weaker weld and it's easier to burn through than plain clean metal.

Agree; I hate using weld thru primer (at least the Eastwood stuff).

I used about 3 cans of Eastwood's internal frame coating though. I Like the spray nozzle it comes with

Our guy uses upol copper weld through. It needs to be put on as a light coat. More is not better. Many times it’s months or more from when a panel is welded to primed and painted. I’m told the primer acts as a sacrificial metal so even when it’s not coating all the metal, it’s proximity is helping to protect it.

This is why this site is awesome.
Thanks for the education guys. I was preparing to purchase more supplies . You fellas saved me some $.
Eastwood is dangerous for me...... it’s right up the road from me and
too easy to spend money there. They do carry some nice products.

Hi Brent,

I was going to ask you how the copper stuff you used worked. I use 3M weld through. My guess was is it is a cold galvanized type primer. One can lasted for a long time and it does not seen to burn and catch fire like regular primer. It is very high in price.
I have welded galvanized metal before and it bothered my throat, this stuff not so much but I weld with a lot of ventilation in my shop.

It welds every easy for me on the edges and I do not think regular primer would work like that. My can was so old that I had to remove the paint from it as the air had gone out. I guess 10 years old. So now I put it in a different container and put it on by brush. Lots of solids in a metallic like finish that dry’s super fast.

I do not have the answer. Thanks for your time!

Bob B
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I've used the 3M stuff, but I have not been able to see the results after welding. No effects on welding with my mig. My favorite use of the 3M Weld through is on my OEM banana muffler. When my used muffler needed to be painted I didn't want to spend any more money. The 3M stuff looks almost like blasted aluminum so I thought what the heck it's made for heat. It has held up quite nicely for a couple of years on my track car.

I started my restoration using Eastwoods copper weld thru primer. When I re-ordered it, I had several cans clog on me. To their credit, they gave me a full refund. It may have been a bad batch, but I had already moved on to this product:

https://www.lowes.com/pd/Rust-Oleum-High-Pe...s-20-oz/3728409

Much cheaper and available at Lowes.

As far as how effective any of this stuff is, I just don't know. At the time, it seemed like the best way to protect bare metal at the weld joints.

Hi Bob,

I've been using Upol copper and as a weld-thru, it is fantastic. It sprays on easy, doesn't cause any sputter when you weld unless you really glob it on, and doesn't seem to contaminate the weld compared to Eastwood. Eastwood self-etching weld-thru is the only other stuff I have tried, and there is no comparison. The Upol does have a tendency to clog the nozzle even if you clear it by spraying upside down after every use, but I've been able to clear it by spraying the outlet with Eastwood surface prep and wiping it off.

My beef is with weld-thru in general. It seems to me that you wind up with just as much exposed area around the weld as when I just use epoxy primer and scrape the weld spot like Stephen described. The only difference is that the primer leaves a black burn edge, whereas the Upol seems to more evaporate cleanly. But exposed metal is exposed metal. Then I wonder if the weld-thru might work against you by interfering with adhesion when you apply any coatings that seep into the seem that would be best in a direct to metal application.

Now, if the purpose of weld-thru is to provide temporary protection until the car is sprayed in primer and seam sealed, I could buy into that. But I'm lucky to live in a dry climate where my bare metal panels treated with Ospho/Jasco haven't shown the slightest signs of flash rust even after months of exposure. If I was still in the humid Midwest, I'd probably continue using weld-thru. But I'm not, so I think I'll just start coating the whole piece in epoxy from here on out.

It all depends on how you use it.

For inside flanges and mating surfaces its great. It encapsulates each weld location. Zinc and copper melt at low temperature which allows it to burn off and the rest will flow around the nugget. Zinc also acts as a sacrificial coating. Copper is more of a barrier coating and less toxic.

In my opinion most use too much zinc. Its not designed to be used like paint. It does not need to be put on thick. That's where trouble starts. For a plug or spot its a good idea to break/ scratch the coating with an awl, nail point, screwdriver, grinding etc to obtain good conductivity down to base metal. During welding if you are seeing green arc popping sputtering welds you probably applied too much zinc or did no prepare the area properly.

In the end a coating is required in areas you will not be able to access and clean well after welding especially flanges or closed boxes.

Rule of thumb if air can get to it its going to rust...
Post welding on inside flanges will burn off some zinc or copper. For interior longs, rocker panels post weld up good practice is some type of interior coating like cavity wax.

On my longs after I welded the Brad Brad Mayeur
Outer reinforcements I had the body on the rotisserie and I put a few cans of Eastwood Internal Frame Coating 14 oz. with long Spray Nozzle – Green (I bought a case) in each side and pushed it in and out and then rotated the body around letting the paint coat the inside of the boxed longs. 2 cans from the back and 2 cans from the front. Then round and round. Bringing the car back to top to the sky and the extra paint poured out of the weep holes in the bottom of the longs. What a mess but I think I got it.

Bob B

It works well that way Bob.

weld thru primer is a marketing masterpiece .

Here is why

a weld pool is approx 2800 degrees when welding on Steel. So how exactly is a paint or anything for that matter going to withstand that heat and stop rust? Well it can't in the HAZ but probably does a great job outside of the area .

I already sprayed my longs with epoxy primer while they were apart, but will treat them with internal coating after the last welding and grinding is finished. I'm trying to decided whether to use Eastwood internal frame coating followed by 3M cavity wax after it cures, or just cavity wax.

As I said, I'm a huge fan of cavity wax. 30 years ago, I made a parttial repair of one of my longs and treated it with cavity wax. Back then, they didn't have the wands with good spray patterns, so I rigged up my own which was a long piece of tubing plugged at one end with pinholes in the sides. When I cut open the longs for the recent repairs, I was able to inspect. My homemade wand provided spotty coverage. But every place the wax hit was still pristine while the uncovered areas had rotted to dust. The new cavity wax forumula and wand kit gives much better coverage and there is a youtube video demo.

But I'm feeling like the best protection would be to treat with frame coating the way you did first Bob. Then cavity wax.

Well, in that situation, iron will sacrifice itself to protect the copper. Zinc cold galvanizing should work in theory, but there is a lot of electrical insulation in the resin binder, and if you don't have the resin, the zinc just falls off. I have used them, but I am skeptical.

Thanks for all this information guys, I am just about to do some welding myself and I am going to use the weld through primer as well.

it's funny how, by coincidence or what ever, posts come up that are germain to the project I'm working on at the time.

lets hope the next thread isn't about genital warts

No doubt!