So I was looking through a magazine, and I saw a cool little saying someone had engraved on an AR-15 ejection port cover. I have a Zastava PAP and wanted to get the saying engraved down the left side of the dust cover. I have a friend who owns a jewelry shop and says he's willing to engrave the saying on my rifle for me. My question is, after he engraves, what do I cover the engraving with to keep the elements out and my rifle from rusting? I searched online and I didn't really find anything that is approved for use on a firearm. Any help is appriciated.