I have used the Brownells 3.5% Ni. rod with a Tig rig on several guns. It has always been visible under hot bath blue. It disappears under cold rust bluing as has standard mild steel rod. I even tried Mig wire(ER70S-6 with TIG)on one job and it worked as well.
It is possible that an expert hot bluer could do a better job with covering the weldment, but the local gunsmith I used could not do it. I did the rust bluing myself using Laurel Mtn. solution and had no problems. I have often wondered if Carbonia blue would cover, but have not tried it. Turnbull's shop did not know either.
OB
