The usual practice is to take the gun apart and remove the blueing before engraving. This may sound strange to you but I use a toilet bowl cleaned brand name the Works that will remove the bluing. Enjoy your engraving adventure
To prep practice plates I buy cold roll steel 3/16 inch thick from speedy metals. Much lest slag, then I use a palm wood work sander with 220 grit stick on disc’s, then move up to 400 grit and stop. MTC YMMV
Or do what I did call the compressor manufacturer and request a MSDS sheet for the oil. They are required to provide one. I ended up buying a 5 gallon can for I think $120.00 delivered. Some where here is a thread on this subject. I have a silentair compressor
Yep I have screaming in to the wind for over 40 /years about the need for shop classes in high schools. Now a few educators are coming to realize college is not necessary. We have a local school that specialized in teaching trades, there is hope.
And the three Rs