two questions about using weld thru primer, (assuming that proper usage is spraying primer on the backside of two adjoining pieces so that primer covers the bare metal that cannot be accessed once welding is completed);

1. Do you change the weld settings on the welder? It welds terribly with poor penetration. Or do you grind off the primer from the anticipated weld spot? If so, then it's defeating the purpose of weld-thru primer, is it not? it leaves bare metal.

2. After welding is complete, do you grind off the exposed weld-thru primer and replace it with POR-15 or primer of choice?

My experience with weld through primer has been that it makes for awful welds. So my use of it was to mask off areas that are going to be welded and then spray it everywhere else.

Brett

Try putting it on a little thinner. It helps.

And don't inhale the fumes while welding ...

I have found that inhaling the fumes improves my welding. Who knew?

From what I have seen, people spray it on what will be the 'heat affected zone', and then use a flatened drill bit to remove it from the exact spot where the weld will go, this way keeping primer on the metal that will not be covered by the weld.

I found that inhaling fumes makes my outlook on everything different.

The fumes are from the zinc in the primer. Burning the zinc gives off small amounts of Chlorine gas FWIW

I spray on the primer, then grind off the spot welds I an going to weld.
I feel better that there is some primer on the back side.

If I am doing a seam weld, Its all bare anyways so I can work the joint from both sides and hammer dolly it flat, grind it, then prime it when done.

The spot welded seams, like all over our cars, is where I use it.
Never use it anyplace else.

Cannot weld through it....
rich

The best method is to coat both pieces that face each other that you are welding. Scratch, grind , sand a small margin to bare metal where the nugget is going to land. I also clean the area on the back side of the punched or drilled hole. Good clean metal to metal contact.

1) First think clean welds.
2) If you are seeing green flashes of arc light you were born in the 60's or 70's, did not have a clean weld zone and are burning zinc up. It looks bitch'n but its really a stellar way to contaminate welds.
3) The cleaned nugget zone should be a little larger than the pilot hole. I normally clean about a dime size area to get good cut into the lower panel.

Zinc and or copper weld sprays melt at very low temperature. The concept is to have a clean weld and let the weld spray just melt around the weld area.

Welding through a dissimilar metal to join two similar materials well would be like strippers sitting on your lap because you think they actually like you. The word "Strippering" originated from that false belief from sugar baby hell that $2 buys a nut and also applies to many things in life including welding. Don't do it, its a trap!

Everything cools down and the weld spray makes a metallic seal around the weld zone theoretically encapsulating it. The second life of weld sprays to is to provide a sacrificial coating between the joined panels to retard corrosion . Zinc or copper will oxidize much slower than steel buying time as a barrier by chemical reaction.

A few good tips: You don't need two or three heavy coats. One even coat is fine.
Zinc is pretty much heavy metal so use adequate ventilation or push the fumes away with a slow speed fan. Green arcing is not so good, the weld is stopping and starting by resistance while contaminating itself. White crud on your welds that looks porous is exactly that.

consider this expert advice
Ron Covell advice

"Q. I'm doing my own paint- and bodywork, Should I use weld-through primer for patch panels, or is it better to use a rust inhibiter after welding by dipping, or sealing, one side of the joint and flooding the other?

A. The answer to your question depends to some degree on the type of welding you do. TIG and gas welding are pretty sensitive to surface contamination, so these process would not be a good fit with weld-through primers, in my opinion.

If you are MIG welding, which is much less sensitive to surface contamination, you can make a case for using a good-quality weld-through primer. I've always preferred TIG welding, so the approach I'd take is carefully sealing the joint after welding."

You can take anything Jeff Hail says about this process to the bank.

BTW if you have an afternoon to kill read his build thread, I learned more in that thread, and another one on Garage Journal, than I have in a life time of doing stuff wrong.

no doubt! Thank you so much for all the helpful replies. I guess I assumed "weld-thru primer" meant I could weld right on top of it, but I guess not.

Good information to know! I'll make sure I have good clean metal from now on.

Thanks again!!

I have some brush on stuff that is the king of splatter. Stick with a good spray like 3M or Copperweld.

I use Wurth

Click to view attachment

Zinc Rich Weld Through Primer
Description
13.53 fl. oz. Aerosol
Features
- 98 - 99% pure zinc content
- Long term cathodic protection for all metallic surfaces
- Adheres reliably to all metals
- Heat resistant up to 914ºF
- DO?NOT apply filler after application
- Good electrical conductivity
- Special epoxy resin for better adhesion

914 degree heat resistance is why I use it has to be an a sign from the 914 Gods