I have done a fair amount of gun welding with TIG and can comment on the bluing aspects. I have found that rust blue will cover very well on 4140 and similar chrome-moly parts welded with Brownells 3% nickel rods. Occasionally, if held up to the light just right, you might see a slight tonal variation. A thin oil film usually make it invisible.
On mild steel like a Win '92 or Mod 61 receiver, the welds will show when hot blued and it doesn't matter what you use for filler rod. If you rust blue the same guns, the welds are truly invisible.
I can't comment on Carbonia blue coverage, but I wish someone would. Turnbull couldn't or wouldn't offer any assurnaces.
OB
