After my first attempt using some overly-expensive copper/zinc based stuff, I took a more scientific approach with some new stuff.
The goal here is to get a coat of primer in between two faces of material that are then welded together (typically spot/plug or lap welded). Then the primer protects the two inaccessible faces from developing rust due to moisture collecting/forming between. The accessible would be painted as per normal.
I prepared three pieces of angle-iron and three pieces of 22-ga sheet metal, each in exactly the same way. First they all got a dry wipe-down. Next, I used PPG Acryli-clean with a fresh towel. Finally, again with a fresh towel, a full wipe down. The numbers above each pair (and corresponding numbers on the sample back-sides) indicate the number of coats.
First coats, with a five-minute flash time.
Second coat, again a five-minute flash time.
Final coat. Then they all sat in the same area for 15-minutes, low humidity, temperatures in the 70-75 range (my guess). The un-coated "control" sample is wrapped up inside the blue paper.
Welder setup for all three samples. Total drying time was maybe 45-minutes to an hour.
Control (no WTP applied) sample clamped up and ready for tacking.
Control sample fully welded.
Back side of the control, nearly full penetration of the 1/16 angle-iron was achieved.
The two-coat sample was prepared with this razor-scraper. I removed about 1/4-in of the primer in front of the lap. This is necessary to get a clean arc-strike. In a real situation, this would be the outside (accessible) side of the part, so re-priming and painting the weld bead and scraped area is possible. The critical side is facing down in the pic, the side you wouldn't be able to paint later.
Two tacks down. Notice the discoloring of the primer. There was a bit of smoke, but not intolerable. I had to step harder on the foot-pedal than the control sample, as evidenced by the just turning blue oxide layer on the sheet metal that's not present on the zero-coat control.
First half of the stitch-weld, more smoke, more discoloring, but nothing ever got wildly out of control like with the "Copperweld" brand I used before (link above).
Two-coat sample, weld complete. Notice the heat-affected-zone, it runs about a half-inch wide up on the sheet metal.
Under-side of the two-coat sample, a bit less penetration than the control. There's some odd "drips" coming from the joint, but they're only on the surface of the primer. Similar discoloring of the primer as on the front-side.
Went at what would otherwise be the inaccessible surface, with the corner of the razor. Light pressure results in light surface-scratches. The primer stays put and doesn't flake off or gouge.
Extreme pressure (corner of the razor is flexing quite severely), same result. The primer stays put, only minor surface scratches are produced. I've got to admit, I'm quite impressed by this.
Four-coat sample preparation, scraping the 1/4-in weld-bead landing zone.
Two tacks down, LOTS of smoke, heavy discoloring, and quite a bit more heat was needed (basically, peddle to the floor).
Middle tack needed so much heat, it started to burn through the sheet metal.
Weld-complete, notice the bumps and rough surface of the primer (it was bubbling). You can also see how much heat I needed to put in, the blue oxidation is all the way at the back-edge of the sheet metal, about 1-inches back from the weld.
Back-side of the four-coat sample. Similar penetration as the two-coat sample, maybe slightly more. The primer is much more discolored, but stayed in place.
Went at it with the razor. Surprisingly the discoloration scraped off easily but not the primer. It stayed put, nearly identical to the two-coat sample. A lot more effort was needed to scratch through to metal.
Here's all three samples in good lighting. You can clearly see how much more heat was needed across them.
Note: Respirator is absolutely required with this stuff, unless you enjoy being sick. The WTP is "Seymour" brand, "PBE, Professional spray paint".
Close-up of the label showing the part-number and details. The back-side has the standard spray-paint bullshit instructions, and mentions nothing about flash-time, dry-time, or recommended number of coats (hence the experiment).
Conclusion: 1000% better than "Copperweld". A stark contrast even, because this stuff actually works. The noozle has a fan spray-pattern which is adjustable to any angle. The stuff sprays on pretty heavy out of the can, definitely not a "fine-mist" like you get with common spray-paint. Since it welds fine with TIG, there'd likely be no trouble at all using other processes.
Interestingly, it seems to stick on better after heating the metal. Before welding it was easily scraped off with the razor. Easier than scraping off a piece of painters-tape, if that's any indication. After welding though, it seems quite tenacious and durable, abrasion-resistant even. Based on these results, it seems like the 1-2 coats range is about ideal.