I'm building a stainless exhaust system, without realising some of the straight pipe i've been supplied is 409. To compound matters, I've used 316 filler (intended to use 309 but grabbed unmarked rods). Reading online 409 is a low grade stainless and pretty much loses it's corrosion properties when welded. No matter how many amps I used or how clean I had it, it gave me grey overheated looking welds. IS this stuff just cheap and dirty to weld, or is the wrong filler to blame. ?