weld thru primers?

I use it as described above where you "clean" the hole. I use a spot drill bit. The style that looks more like a little end mill with a tiny point. Some brands of weld-thru are better then others and some are total junk. U-pol is my choice but not cheap, Medallion if I can find it, 3M brand I don't like, and the $6 can of "cold galvanizing compound" leave on the shelf at Lowe's. They all seem to contaminate the weld to some degree even when you clean the hole. In terms of long term durability and corrosion protection I haven't noticed really any difference on cars I've seen long term between prepping with weld-thru or just taking a spot bit and cleaning the e-coat off the underlying panel in the hole. If you have to prep between panels to clean up the drilling and whatnot it's better then nothing I suppose.