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Old 07-21-2015, 08:14 PM
longyard longyard is offline
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Default Weld Through Primer: Issues

In working on the Duetto floor boards I used weld-thru primer wherever necessary. At first, I was using Spray Max 1K and had NO issues using my spot welder, or MIG plug welding.

AAAAWeTh1.jpg


However, I ran out of Spray Max 1K and switched to what I thought would be a better product, SEM's Weld-Thru Primer and I began to have many issues with making strong spot welds, and even plug welds. The SEM product also was grainy, like it had very fine sand in it.

AAAAWeTh2.jpg


Can anyone recommend a weld-thru primer that's better than Spray Max 1k? Any comments in general about using weld-thru primers?
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Old 07-21-2015, 08:29 PM
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Marty Comstock Marty Comstock is offline
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NO weld through primer is best.

Better off using epoxy primer, then scraping out where the rosette weld will be with a flattened out drill.

Nothing in a spray can can match the adhesion urethane has. I have seen that garbage peel right off a well prepared surface,

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Old 07-21-2015, 08:51 PM
longyard longyard is offline
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You're killing me, Marty! I thought I was doing it the right way.
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Old 07-21-2015, 08:52 PM
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MP&C MP&C is offline
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Bill, epoxy the two sides, let it cure, then plug weld. I have done the same with the spot welder, let me dig up some more pics..


Spot welding:

Quote:

In order to use the spot welder along the bottom seam of the lift gate, we needed to clean off some epoxy primer where the spot welds would be located. Used and even spacing of 2" and marked both flanges...





In order to remove as little epoxy primer as possible, we decided to use the modified plug weld drill bit (the flattened one) rather than the errant marks and excessive paint removal that a roloc sander would make. In order that the flat drill bit didn't walk all over the place, another specialized tool was made....









Here's another close up of the modified bit...



.....and the "prepped" flanges...





Bottom flanges spot welded together...

Plug welds.. (repeat post to help with the topic at hand)


Quote:

When assembling adjacent panels I'll typically just use epoxy between the two, as shown here, painted on the rear tailgate opening, and also on the underside of the tailpan. This provides rust protection where in many cases the factory process does not add any paint until after assembly. In cars like this 55, in many cases the inner voids such as in the rockers or floor crossmembers never receive any paint finish inside, making them prone to moisture and rust issues.

Where this epoxy will not allow welding, you will need to clean the paint off the adjacent panel within the drilled plug weld hole.





Which is what brought about the need for this tool, the same size drill bit I used for plug weld holes is flattened and backfaced to resemble an end mill cutter.








This shows how it works, cleans the paint from the metal surface, but having a flat face on the cutter, it doesn't affect the metal as a regular drill bit would. Notice most of the "chips" are paint....





And welded......





Some guys will use a weld through primer to accomplish this same process, I prefer the epoxy as I read an issue of Auto Restorer magazine quite a few years back that a study showed epoxy offered better long term protection from rust. I'd say use either method, as something, no matter whether you choose weld through or epoxy, will be better than nothing at all.

For application, I spray inside my paint booth. For a small application like you see here, I'll mix up some epoxy and brush it on. (note brush strokes in first pic above) Again, the application process won't matter much as any exposed epoxy will be sanded and re-applied later, but now that we have some epoxy between the two panels, there is better rust protection regardless of how it got there.


Back to the weld through primers, I believe they operate on the premise that the heat liquefies the paint around the perimeter of the weld where once the weld starts cooling the WTP seeps around the weld to seal it. All that aside, I agree with Marty, there is no way in my mind that an aerosol paint can match the endurance of an epoxy.
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Last edited by MP&C; 07-21-2015 at 09:21 PM.
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Old 07-21-2015, 09:42 PM
skintkarter skintkarter is offline
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Yet another great tip Robert - now I have a use for those slightly rust pliers in the junk tool box! Pays never to throw anything away
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Old 07-21-2015, 10:29 PM
crystallographic crystallographic is offline
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Robert,
I've drilled out a few spot welds and I've dealt with plug welds and WTP chemistry for a long time. I have to say that your "end mill drill" combined with the end milled pliers suggests "months of heavy drinking" to come up with.

Nice clean approach to a nasty problem. Thank you.

Foo cans are NO match for real chemistry delivered by real paint guns
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Old 07-21-2015, 11:43 PM
longyard longyard is offline
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Absolutely outstanding Robert. Thank-you so much!
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Old 07-22-2015, 05:35 AM
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Robert has some Bloody good ideas.
What a Champion.
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Old 07-22-2015, 05:40 AM
Oldnek Oldnek is offline
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But To answer your question Bill, I use the U-Pol #2 Weld through Copper primer, works great. The one in the green can.
The Epoxy I use burns like No Tomorrow... Even when plug welding.
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Old 07-22-2015, 08:23 AM
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John, the epoxy I had been using (as shown above) was House of Kolor, and the extent of the "damage" from heat seemed to be limited to discoloration. I have had some plug welds that turned the area even darker than that shown above, and after letting the area cool completely I did a fingernail scratch test, with no marks left in the paint and no loss of adhesion whatsoever. I have some upcoming plug welds using SPI epoxy, so I'll be sure to update results with that primer.
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