Weld Thru Primers - What Do You Like?

[DeathProofCuda]

Are there any weld thru primers out there that are clearly better than others?  What do you like?

[Filthy Filbert]

As a weld engineer and metallurgist, I can say that I don't like any of them.   Nothing but pure clean rust free, dirt free, grease free metal. 

Some flux core wires will have fluxes in them that help clean dirt, dust, rust, etc away from the liquid metal and collect it into the slag that forms, however, If you're not using flux core, and just a good ole "catch all" wire like ER-70S-6, I wouldn't use any weld through paint/primer

But...I know that weld through primers get used all the time. 

[gzig5]

Hated the NAPA zinc based stuff, but to be fair the area I was welding was really difficult to get clean.  U-POL zinc worked pretty good and their copper based was better still.  Haven't tried any others.

[DeathProofCuda]

As a weld engineer and metallurgist, I can say that I don't like any of them.   Nothing but pure clean rust free, dirt free, grease free metal. 

Some flux core wires will have fluxes in them that help clean dirt, dust, rust, etc away from the liquid metal and collect it into the slag that forms, however, If you're not using flux core, and just a good ole "catch all" wire like ER-70S-6, I wouldn't use any weld through paint/primer

But...I know that weld through primers get used all the time.

@Filthy Filbert

As I understand it, the purpose of weld-thru primer is to protect the pure clean rust free, dirt free, grease free metal along flanges or seams that were being welded together and will not be accessible for paint, not to clean less than perfect metal.  For example, you spray it on the bottom section of floor pans where they will be spot or plug welded to the frame rails or torsion bar cross member.

Craig Hopkins of the AMD installation center swears by Medallion RS-512, but I'm assuming there's a sponsorship relationship there...

[Rich G.]

I've used the Napa and the Eastwood weld through primer and didn't like either one. I wind up scraping the primer out of the punched hole I'm going to plug weld and then weld it. Maybe it's only good if you're using a pro spot welder and not a mig.

[Dmod1974]

The Eastwood weld through primer is garbage.  Messy, and can be scratched off with a fingernail even with proper prep and dry time. 

I used SEM Copper-Weld for most of my build and thought it was several times more durable and scratch resistant.  It welded pretty good too.  U-Pol copper primer is similar, but much sprays out much thinner.  It welded fine as well.  Since the whole idea is to prevent rust in the weld overlaps, I prefer the SEM product since it seemed to be more robust after drying.

[Filthy Filbert]

Craig Hopkins of the AMD installation center swears by Medallion RS-512, but I'm assuming there's a sponsorship relationship there...

He is also using resistance spot welding.  Totally different than Gas Metal Arc Welding.

Not saying that there aren't any good ones out there, also not saying that every welding code prohibits it. Just saying that one would have to do a lot of proof testing to show me that they are able to use it and not compromise the integrity of the weld. 

Softer metals like zinc or copper, with lower melting points, can create a type of cracking phenomenon that's similar to water seeping into pores in concrete and freezing, ripping it apart and causing cracks. 

Not to mention all the other stuff in the primer that can cause porosity in the weld. 

Usually, with a resistance spot weld, those softer metsls melt first, and the pressure from the electrodes push them out away from the spot weld nugget long before the steel reaches melting temperature, helping to prevent this type of cracking.

Like I said, RSW is totally different than GMAW; and as you can tell by most of the other replies, there's also a lot more guys who don't like them at all, than the guys who do.   

But, to each their own.

[DeathProofCuda]

Craig Hopkins of the AMD installation center swears by Medallion RS-512, but I'm assuming there's a sponsorship relationship there...

He is also using resistance spot welding.  Totally different than Gas Metal Arc Welding.

Not trying to start a pissing match, and I appreciate your input, but Craig uses both RSW and GMAW.  He may typically use RSW at the AMD installation center, but he's done a bunch of how-to videos where he is mig welding and using the same stuff.

[DeathProofCuda]

Pretty interesting video here posted by 3M about applying weld thru coatings.

https://www.youtube.com/watch?v=y317f2KP-z4

Ironically, they say not to "weld thru" your "weld thru" coating.  Also pretty interesting that if you look up the tech data sheet for 3M weld thru coating it doesn't say anything about not welding thru the coating.

[Dmod1974]

Industrial welding and car welding have different requirements though.  I'm not aware of one OEM that does not require weld through primer in places getting MIG welded and weld through seam sealer or panel bond in areas that will be spot welded.  Yes, there are seam sealers designed to be spot welded through, and panel bond is as well.  If left in bare metal those seams exposed to the elements would rot out in a couple of years just like they used to on the older cars.

I don't think anything will survive the MIG weld pool and still seal, but the primer still protects the rest of the flange and area surrounding the weld.  Some people forgo the weld through primer entirely and use an epoxy primer on both panels instead, scraping the primer away in the plug weld holes to ensure a clean weld.

[soundcontrol]

I tried a bunch of them, the zink based stuff sucked. The copperbased works good, but took forever to dry, and I still have to clean out the hole a bit before welding, I did redo a weld (rip it apart again) once after using the copper stuff and it seems to creep in close to the weld pretty good. Then I tried Volvos primer, the blue one, that was the best one so far, dries quick and I can weld in it without too much hassle. Best procedure so far is, (if possible, depends on location) the trick not to make a complete hole before, I drilled almost thru the first layer, then welded (primer in between). Works great.

[torredcuda]

I use the SEM copper stuff in a quart can and brush it on as I had issues with the spray cans clogging and wasting expensive product. As for affecting the weld you can`t weld copper so it has to burn away before the actual weld starts happening so is only going to protect around the weld area. Like others have said after coating I clean as much of it off the actual weld area so the weld starts easier with less spatter. I`ve never had an issue with weak or broken welds and even though they are technically structural we don`t need to worry about following welding codes, I would guess most mig welded spots are as strong as factory spot welds.

[Jocigar]

I've watched a bunch of youtube videos trying to research, they were all put to the test but also tested along side rustoleum primer and rustoleum high heat bbrq paint, my personal take away was that the difference is minimal in some cases better considering an $8 product versus $28, I'm going to try the high heat bar-b-que     

If pinch welding I can see the necessity of medallion type conductive primer.     

[Jsand73440]

I bought a can of the SEM copper weld thru  primer a few weeks back, i got screwed over on the price and had to pay retail at $34.00 a can!

[soundcontrol]

If you can find this in the US/Canada, try it, the best I tried so far.