I am aware the name is a complete misnomer, you can neither weld through it or use it as a primer you can't paint over it, it is supposed to have a very high concentration of zinc particles which is sacrificial and a no nonsense binder and solvent which won't pollute the weld pool. Used in a well prepared lap joints but you know there will be water ingress e.g sills. Went to a auto paint supplier and asked for a rattle can, got home and read the fine print, it's an anti spatter paint, took it back they said ''we use it as weld through primer all the time'' I said but it's got no zinc or even copper in it, ''ooh yeah''. UPOL make one that I think Halfords no longer stock with their other products. I suspect it's this confusion over '' Weld Through Primer '' ( WTP) in my naivety and early days I believed it meant what it says, so the bad results. So I stopped using it and everyone in the same boat did the same. I have bought a can of Halfords own brand WTP which states ''suitable for welding through'' it can't mean what it says so which WTPs can members on this site recommend if used properly and maybe call it ''Sacrificial paint'' not the same ring I know.
Bert
Bert