If the above is true, why would it matter about scootching the SN around
What you are describing is the method licensed Firearms Examiners in forensics units use to "recover" obliterated s/n's. It is a laborious process that is gone through in a lab to retrieve it. It is not 100% effective, and it irreparably weakens and damages the frame (powerful acid is used in the process). Mere possession of a Firearm with an obliterated/altered s/n is a serious felony, and not worth the risk.
Example: Last year I found a correct and intact WWI 1911, with all correct parts and military proofs. Someone had, however, removed the "US Property" from the frame and altered the S/N. I consulted with the local ATF and did legal research, and it was still a serious felony to own the gun. I investigated restoring the S/N using the acid etching process (or any other technology, such as x-ray crystallography), but it would have wrecked the frame in the process of recovering the original s/n and re-applying it, and/or been prohibitively expensive.
I can understand the OP's desire to "clean up" the appearance of his Ruger. Lord know's why they care so little about the appearance of their guns that they stamp up the barrels and mark up the frame with the S/N. Because of that, the "newest" Ruger I have was made in 1973, before they started that.
THe court case JohnnyDollar cites above isn't relevant, as it is about obliterating, with the intent of hiding, the original s/n. It might be possible to get permission from the BATF, but you'd better have that in hand before doing so, or your work could land you in jail for your effort.
