I see some otherwise fine old guns that have numbers or the like etched (usually poorly) in the metal with an electric pencil in some prominent location. Is there any way to remove, disguise, or otherwise mask these, short of a complete re-blue?

I have never heard of a cheap, easy fix. Polish out the damage and refinish is the most obvious solution.

Some simple pawn shop engraving scheme might hide it, but then you have another detractor to its originality.

depends on where it is really...if on the backstrap or grip frame, can obliterate, polish out and touch up with a bit of cold blue,this will ONLY be 'temporary" as there is NO "permanent" type of cold blue....needs to be buffed, polished and reblued (refinished) to do it "right"...if its on a 'flat', ala the side plate, oh well, BAD news.....if its a social security number, at least "alter it" or obliterate it, stipple or 'something'......if its a "presentation engraving" , leave it be, thats part of its 'history'......
Good luck....

and Merry Christmas..........................

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dan
NRA Benefactor,PMA