I have a pretty interesting question that my brother-in-law posed to me last night...
He is a gun collector and restores old guns that previous owners have drilled for scopes, etc.
On late 1800 to early 1900 Winchester sporting arms, he has had weldors in his area fill the non-factory holes with TIG. Then he takes the factory blueing down to bare metal and then re-blues the entire gun. The repair cannot be seen in bare metal, but is very evident after blueing. Plus, he says that the filled hole is a harder metal than the proof steel of the rest of the gun.
So, anyone run across this? What would be a proper TIG filler to take the blueing like the rest of the metal?