The Stovebolt Site Forums The Shop Area Paint and Body Shop Weld thru primer

Does weld thru primer provide a moisture barrier? Thanks, Scott

Do you mean 'will the metal under weld through primer rust if it gets wet' ?

Yes; (skipping over all the variables of corrosion prevention) it can rust both before and after welding.

Weld through primers need to be top coated to seal or provide a barrier for the metal under it. Never leave weld through primer exposed to the elements thinking it will be rust-proof.

Brad

My opinion of weld thru primer used on our trucks isn't going to prevent rust from forming in the area of the welds. While it is conducive to welding (no contamination), it is destroyed near the welds, providing no rust prevention there. Since the locations we are welding are often multiple spot welds, 3/4" apart, that leaves a lot of bare metal exposed underneath. I don't sweat it much because I use epoxy primer after the body work and that should help keep the rust at bay for a good long time.
Carl

It makes me wonder, what is the purpose of this product? It seems like it would be an unnecessary step if it provides no protection. Thanks again, Scott

The purpose has something to do with a fool being seperated from his money I believe.

Any good epoxy primer will work better at protection than weld thru. Just remove what you need for welding and then reapply when your finished.

It is typically used only between the metal of a lap joint that is then spot or plug welded.
Supposed to help prevent rust in that lap, and while it does burn away at the weld (as will any coating) it protects the metal in the immediate area. The idea being it is less heat sensitive than other primers and will remain and protect closer to a weld.

It is also an excellent way to contaminate a weld, so "weld through" is misleading.
If doing plug welds it's a good idea to scrape/clean it out of the hole you're welding, and a little more outside of that if you can.

When I worked at a vintage car restoration shop we used it often, thought it necessary, and we despised it too. I made up a spoof paper label and wrapped it around the spray can. It read something like "Spray on Weld Contaminator... best when used liberally".

For exposed metal there are lots and lots of better primer choices.

Grigg

weld thru 'primer' [high zinc solids] is most useful on the back of welded areas that aren't accessible for finishing after welding, zinc is a very good rust proofing, as shown by galvanized metals - just be aware you don't want to be inhaling the fumes created

Bill

I am with your red58. I put it on where I am welding patch panels and closing up an area. I figure it's better than nothing and better than just a regular primer.

My question is why is a rattle can of zinc paint priced like it was full of gold? Didn't they start filling pennies with zinc to save on cost?
Carl

yes Carl, zinc is a lot cheaper than copper, but even with that change in the penny, it's produced at a loss
high zinc solids primer or "cold galvanize compound" isn't "paint", it's 90-95% zinc in a carrier .... if you want to use the best when replacing body metal, use JetKote [galvanneal], a specially galvanized metal that is what good body shops use nowadays, zinc is an excellent rust proofing

Bill

I use epoxy primer, per a restoration publication I read quite a few years back it offers better long term protection. Regardless of what you use, I think you'll find as stated above that weld through is more of a name and less of the process of how it's actually done. Too much around the weld area will contaminate the weld. For cleaning paint out of the plug weld hole, I use a modified bit that's been flattened and backfaced to resemble a milling bit. Works well to clean the paint with minimal "metal removal" from adjacent metal. See here:

http://www.garagejournal.com/forum/showpost.php?p=3063179&postcount=181

thanks for the tip Robert

A copper or zinc weld through primer doesn't burn away from around the weld site causing rust to start immediately.

To not use it only invites rust.

One sure way to avoid this is to not lap weld and another is to simply glue the lap weld with a panel bonding adhesive like Fusor. Bonding panels is a proven method of incredible strength without dealing with warpage and other heat related symptoms due to welding.

How will it do on a surface where I used fast etch, under the outer edges of the rear fenders? I am welding a formed round reinforcement in that area to stiffen the fenders and wanted to protect it in that area. Thanks, Scott

I have used weld thru primer on my '52 cab, and I will admit that I did not take it apart to see if the primer was still there after welding. 200 spotwelds!
I don't have any of that liquid gold left to do a test to verify what you are telling us. I certainly hope it was worth it for the cost of those rattle cans.
Thanks for your input!
Carl

While most of the vintage tin around will have left the factory some 60 odd years ago with absolutely nothing between the panels, something has got to be better than nothing at all. But for longevity, which would you think will hold up better, a non catalyzed finish out of a spray can or a catalyzed epoxy finish? Which do you think will offer better adhesion to the bare steel panel? I've got some panels here that I've done hammer and dolly work after the SPI epoxied finish was applied, with no detriment to the paint adhesion.

As far as the present day manufacturer's recommendation for using the weld through in repairs, most body repair facilities will not take the time to spray a good epoxy finish between a lapped metal weld seam as they would need to mix up paint, clean a paint gun, and allow for cure time. You'd have better luck getting anything actually used at all if it was EASY and FAST. Spray cans are easy, and if you do some research into both this and I-CAR recommendations for other repairs, you find that much of the guidance given is less about being the most optimal method of repair and more of what is easy or easily duplicated so that they would have better luck in getting something done in the course of a body shop effecting that repair.

Darcy brings up a good point,
Many of the lap joints used at the factory are not because it is the best method, but because it is a faster method of assembly. Faster down the assembly line, faster out the door, faster to rust so you'll need to buy a new one. Lap joints, Pinch welds, or whatever else you'd like to call them are the main source for rust issues in all vintage vehicles. This is why many of those seams have been removed from the 55 Wagon we're presently working in the shop, in favor of a butt welded seam that can be finished on both sides, and epoxy primed on both sides to seal out the elements in hopes this would be the last such repair..

I use Dominion brand weld thru primer and like it. Pretty reasonably priced and it seems to do a decent job on the structural lap joints on the Z we are putting together. Used it for a couple hundred spot welds so far, I've cut a few welds apart after using it and they were ok.