i have not done a gloss scope but i have done a nice blued barrel, i just cleaned it really good with acetone and put light coats on. has stood up pretty well.
If you don't want to go to the trouble of lightly sanding it first, pick up some 'self-etching' auto primer (I found some in a spray can at Wally World) and use light coats, but shake it really well before use...