Weld thru primer or not?
 

Hi all,
I'm in the process of putting in a full new quarter panel, outer wheel house and trunk drop off on my '75 T/A. I'm using a TIG welder. I ground off the black coating on the flanges used weld thru primer while installing the trunk drop off, and it made the welding a lot messier as it contaminated the weld and the tungsten. Can/should I skip the weld through primer and just rely on body caulking afterwards to prevent corrosion between the panels for the rest of the job? Not sure if it makes any difference, but the panels are all from AMD.

I recently tried a few weld through primers, didn't have the results I wanted either, it's still burning the primer off the welded area no matter what, however it might provide some extra protection for whatever piece of metal becomes inaccessible after welding.

Been TIG welding many years, can't imagine that process is going to work well with any kind of weld through primer.

After much experimenting, I found it best not to try to weld through the 'weld thru' primer as like you have experienced, it causes the weld to be contaminated.

What I now do for plug welds where 2 panels attach, is to spray the backsides of both pieces with weld thru primer, then clamp into position. Once everything is in place and ready to weld, I take a very small flat screwdriver and scrap all the primer off though the pre-punched or drilled hole where the plug weld will start.

Seems that once the arc is struck on bare steel, it flows out to the edges of the hole without issue, and the primer is still intact between panels for protection.

Note this is with MIG, I haven't used TIG so not sure how this will change things.

Weld through primer will not work with TIG…..

Don

Weld through primer will out gas and ruin any welds with mig or tig because the out gassing will interfere with the shielding gas. Weld through should only be used with resistance, aka spot-welding. Spot welders for automotive repairs are pricey so few hobbyist have them. An excellent alternative would be one of the panel adhesives for flanges that would normally be spot welded. The sail panel area will need to be butt welded and prepped on both the inside and outside before epoxy priming. Do not try to overlap and bond any outer body seams such as the sail area as a line will bleed back after paint. BMW does have a procedure to bond the sail area but they supply a specific filler to finish the seam with that is not available otherwise.

If I go the panel bond adhesive route, which one? Apparently there are a bunch of them. Even 3M makes 3 different kind.

I use a mig. I'm a rookie but I liked the copper rich weld thru primer. the term "weld thru" is a misnomer though. I drilled and plug welded everywhere I used it like mentioned above. worked great i thought.

its the only way to rust proof between flanges and such, i liked having it there. I seeped por15 into most all areas when done welding to boot.

oddly, this copper rich weld thru stuff has a very pleasant smell.

I’ve used SEM’ s copper weld thru primer. Prior to welding I run a small ball grinder thru the punched hole to clean to bare metal. Seems to flow out much better.

Let’s face it….up here in the Northeast…Factory Built vehicles as old as 2016 are now rusting…we are talking ( rust holes ) . I see it every day. So not using a weld through primer don’t make a damn bit of difference. But I do drill access holes to get some 3M cavity wax in those welded areas from the back side.

yepper, daily drivers are doomed to rust.

these hobby cars however, being pampered, drivin on nice days, tucked away in a garage, ought to last forever. specially if rust proofed to the nth degree during a restore using everything under the sun as rustproofing as you go like I did.

I went ahead and TIG plug welded the outer wheelhouse. I did use Seymour weld through primer. The welds don’t look pretty, but will definitely hold. I should have just skipped the plug welding and made a few lap weld beads and been done with it. It’s not a visible area.

I will use 3M 08115 epoxy for the quarter panel on the more visible joints like the wheel arch.

One thing I’m still trying to understand is why the factory used a brass tack weld where the quarter panel attaches to the rocker panel. Anyone any theories or knowledge about this?

The weld through primer l used actually states in the instructions to remove it from the center of plug weld openings before welding. In addition, it states that it's not actually primer so remove it from areas to be painted also. Kind of makes you wonder what the point is... especially at $30 a can.

I am partial to sem products but have used 3m as well with good results. I would look at the shear and tensile strength as a guide. More psi more better. Follow manufacturers instructions. Done correctly this is the strongest, most corrosion resistant method I know of and I have been doing restoration and collision work for almost 40 years.

When I weld panels on I use stainless wire in my mig. Welds just fine and keeps the weld itself from rusting to some extent.

This is because its NOT arc weld through primer. A mig or Tig is an arc as well as a stick welder.
The only thing it does is conduct electrical current for resistance welding, while having rust inhibitors.
I don't care WHAT the can says. I use Copper " weld through" when needed. Anyone see what a mig does to copper?
Never arc a weld on weld through primer.
At this point, with the strength of Glues, your strongest, and least likely to rust work can be glued, With an overlap.

Tried Stainless and found the weld was harder than regular alloy. This caused cracks at edge of weld and the expansion and contraction was less than alloy. Unsure if that will create an air gap under filler or paint.
Bet its expensive now, mine is a few years old and like other wire I store in food saver vacuum wrap.

Is galvanic corrosion an issue with stainless filler on mild steel panels?

Idk I’ve been using it for close to 20 years now and haven’t had any issues .
Have had issues w adhesives other friends have used . This I my personal preferred Method after many years all the way from brazing to adhesives. This has been the best so far.
Everyone is entitled to do as they wish though
The little bit of extra cost is well worth it to me. It’s grinds a little harder too so keep the welds nice and fiat of you can .

>> GALVANIC<<< >> Any metals,. even if one is a Stainless alloy, and one is mild steel alloy have different "potential difference". This means their ability to accept a electrical charge.
These charges can happen due to static, earth rotation, electrical waves ( Radio, light, etc) .
If they have different abilities to hold a change, one becomes positive, and one negative ( trying to make this simple to understand)
So you have a weak BATTERY and they do corrode as they pass current 'IN A CONDUCTOR" ( even lithium turns to a white dust).
The conductor can be FOG, High Humidity, RAIN, Water, or some acids.
So basically try to keep what you have done / welded, Clean Primed sealed and Dry at the metal is the key.

I gave up on the idea of using glue mostly because I wasn’t confident I could clean/keep the surfaces clean during installation especially because of the way the back of slides over the tail panel. I bought a HFT 240 volt spot welder and welded most of the panel that way—I was impressed that it survived the entire job. This was a major PITA because the welder weighs so much—ended up being a 2 man job. I used TIG where the spot welder wouldn’t fit. The last remaining section is where the sail panel attaches to the roof.

I can painfully fit the spot welder there or I can TIG it together. I’m leaning toward TIG because I’m more confident of the welds. It’s a structural area and I’d hate to see spot welds giving out. Any suggestions?