J
Jose
This is truth. It is typically an acid solution that is used over METAL parts.
From Iowa DPS http://www.dps.state.ia.us/DCI/lab/firearms/restoration.shtml
"Today many of the manufactured items such as firearms, automobiles, motorcycles, & bicycles are impressed with a serial number for purposes of identification. In an attempt to hide the identity of the numbered item, the numbers are often obliterated or removed by mechanical methods such as grinding, peening, and punching.
Serial numbers are usually stamped on a metal frame or plate, with hard steel dies. These dies are applied with enough force to sink each digit into the metal. Restoration of obliterated serial numbers can many times be accomplished because the metal crystals under the stamped numbers are placed under a permanent strain. When a suitable etching agent is applied, the strained crystals will dissolve at a faster rate as compared to the unaltered metal, thus permitting the etched pattern to appear in the form of the original numbers. If the number is ground to a depth that removes the strained crystals, or if the area has been impressed with a different strain pattern, it is usually not possible to restore ."
I don't think yet I have any experience with it. Just from standard blah blah blah textbooks.
Annealing, re-hardening, and re-tempering the metal will make the stress marks in the crystalline structure disappear.
The cops always think they are smarter than everyone. They are usually wrong. It's just that 99% of the criminal element is so GD stupid, they make the cops look like brain surgeons.