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Another weld through primer thread

hello gents

I have done a lot of reading on weld through primers and attempting to prevent seam rust in general. My personal take on WTPs is basically they
are a waste of time, with the exception of Medallion etch weld through primer by Rubber Seal as the best on the market. With regards to a spot weld
epoxy primer on both sides of the metal, cleaning off the primer at the weld is the preferred method to my knowledge. Getting ready to order some
SPI epoxy for my project. I have alot of unibody pieces that were cut away to access rust. Anyone have a better method than epoxy/seam sealed?


Paint Fanatic
Staff member
I question the medallion weld through being state of art as weld through primer is nothing but Zinc and aerosol propellant.
normally 90% to high 90's in Zinc, the more Zinc the less sparks.
Now someone has come out with a copper that all my reading is not as good.

Only a few months ago i read where some of the manufacturers are recommending weld through e-coat or epoxy, I have been told over the years that shops do weld through my epoxy but myself have never tested, so I would GUESS most epoxy's can be welded through but I don't know if this is fact or not.

Zinc has worked for years this way but Zinc has very poor adhesion and should always be sanded off the outer panels before epoxy is applied.

I recently sold my mg welders and torches so I cannot test.
Just my personal observation here but, I have found the adhesion level of the zink wtp to be very poor. The last I bought was copper based and it does adhere better than the zink. With that said, I truly can't say if it would be better to just use epoxy and forget the wtp altogether.

Here's a brief breakdown of current OEM positions on the use of weld-through primers:

Chrysler: NO
Ford: YES
Toyota: YES

My own experience indicates that zinc primers can help prevent corrosion in properly prepared* flange areas that can't be reached by wands or other spray equipment after welding. It's only to be used in the HAZ (Heat Affected Zone).

However, weld quality goes down the more primer is used. Also some brands seem to contaminate the weld area more than others. Some manufacturers specifically warn against the use of copper containing primers:


*Properly prepared means prepared the same way you'd prep a panel for epoxy, sand with 80, then clean with #700 or equivalent.
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Trying to be the best me, I can be
Epoxy both flanges, you can use a roller to put it on with. Let it sit overnight. Drill your plugweld holes, fit up and scratch away the epoxy in the plugweld hole. When finished welding and grinding recoat with epoxy.
Thanks for yalls replies. The OEM stances are interesting, and it makes me ponder what perspective they're coming from when they say yea or nay,
based on my own experience. All the research i've done online including hot rodders forum, the HAMB, jalopnik and others indicates that chris hamilton's method is the preferred method. But i come here yearning for something better if it exists.

There's a feller on youtube (oh lawd), that experimented with corrosion tests consisting of mig welded coupons (seams) using various WTPs and epoxy. I forget his username but his conclusions indicated that basically everything burned away from the HAZ. (wink wink crash) The two products that stood tall was the Medallion WTP and epoxy, with epoxy holding a slight advantage.

Also I don't know if any of you watch Graveyard Carz. They seem to do exceptional work even though Worman acts like a douche sometimes. There was an episode where he talked about their use of wtp. Which is even more interesting considering Chryslers stance on it according to Crash and Wormans gung-ho OEM by-the-bookedness.

Crash, I value your expertise and was wondering if your use of WTP on a properly prepared panel would include both sides of the flange and whether or not you clean the weld hole first. Also I noticed you didn't name your brand.

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An interesting excerpt from the link that crash provided. " When the weld is performed, the zinc liquefies and flows into the weld-pool, protecting the weld from corrosion." It is my belief that nothing can withstand the heat generated at the weld and burns away, certainly not "flowing into the weld pool." Some more than others, and epoxy actually outperformed the WTPs when using the method that Chris Hamilton pointed out, which is what I've been reading.

Do i think I'm smarter than the manufacturers engineers? No. But the inconsistency around the use of WTPs makes me want to ask more questions.
Am I being OCD about this? Yep.
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Paint Fanatic
Staff member
Guys you would not believe some of the calls i get on tech line, example, I was told to use 2k direct to metal urethane primer and do all my body work over it???
Or i went to a paint school (those who can do those who can't teach) mainly to find out how to do my old vette as our shop never does vettes and I'm a body-man. The answer was "use Variprime on the fiberglass" and i got the call $3000 later because it was red and why is the paint falling off in sheets, they said "I missed mixed something"! Yeah right.
1000's of times I have told people you have to know who you are taking advice from or double check the answer before you do something stupid.

This thread is the perfect example of why I love this forum, did we solve the problem or question a 100%? NO! BUT with all the high quality of people on here, I and I think a lot of others have received some very good direction and most important we learned something. KEEP IT UP!!!!
Oh, man. I hated using the weld-thru primer because of the adhesion problems and it just didn't seem to make much difference. In my ignorance, I was a bit enthusiastic using the stuff and didn't confine it to just flanges. I also didn't ensure that it was completely removed prior to recently painting my interior with epoxy. I think I got most, if not all of it, but didn't make sure so I can only hope that my prep was sufficient (every exposed area sanded with 100-grit and hit was a scuff pad) to prevent any future epoxy problems.
I couldn't agree with Barry more! I've learned so much in the very short time since I've started studying up on this whole painting thing. I'm glad I read this thread before I painted over some of it that I have on exterior panels!
I have many brands of weld thru primer that I have tried with various types of MIG and TIG welds (I’m more of a welder than a painter).

I don’t use any of them beyond the initial “maybe this company has the magic formula” testing. The last one I tried was called Like90. I didn’t like it.
The ones I tried, always seem to contaminate my weld and cause porosity. I think on my current restoration I am going to brush on some epoxy primer, weld the panels together and then seam seal the edges . I'm hoping that the combination will keep most of the steel protected and the seam sealer should help prevent water from reaching any exposed steel.
If you can keep the wtp an 1/8" or so back away from the plug weld perimeter(on both halves) that will help with your porosity issue. When you only clean to bare metal in the plug weld hole, the adjacent wtp can suck into your weld puddle and give you fits especially with tig, then mig. Self shielded flux core wire would give the best results in that situation or (6010 or 6011) stick lol
There is a lot of nuance to this subject and mostly I keep quiet on it because my views don't quite align with the majority here. What OEMs really want is for their galvanize/galvanneal not to be disturbed at all, and for STRSW to be used. Where that's not possible, there are particular procedures in place for each model, manufacturer, and area of the vehicle in question. In some cases, I tend to think that an "all of the above" approach is called for, with certain areas getting weld-thru, then all pinch welds being saturated with epoxy first, then body wax once the job is ready for assembly.
Epoxy both flanges, you can use a roller to put it on with. Let it sit overnight. Drill your plugweld holes, fit up and scratch away the epoxy in the plugweld hole. When finished welding and grinding recoat with epoxy.
This is exactly how I go about it now. Tried the weld thru and never could get the welds as clean as I wanted.
Tried the weld thru and never could get the welds as clean as I wanted.
You never will, a contaminated weld is a crap weld any way you slice it.

As far as adhesive on parts I have seen too many complete failures in the use of that. I have personally pulled off, with my bare hands, damaged truck bed sides that someone used that on. Clamp too hard and it becomes useless and bonds nothing, prep it wrong and the same will be the result, total failure. If someone is going to use it they must first know how to properly use it.
Something that gets left out of the instructions, is when using adhesive, or flooding the seam with epoxy or cavity wax, is to make sure there is a way out for the water that gets inside of the panels on cars not designed for its use. Not aways an issue, just something to keep in mind.

I have also separated several seams on older cars that had a lot more rust above the seam, than in the seam itself, so its important to make sure the original factory seams have an exit and factory drain holes are open.
I have a question on this older topic.

It's about using the epoxy in place of WTP.

When you apply it(epoxy) by roller on the seamed to be spot welded, do you still need to mix the primer? Or just use it straight out of the can, I'm talking about using the Spi.

Appreciate the feed back