Wiping with denatured alcohol to remove any surface oil is a good start. A little lacquer thinner will usually cut the gloss on stock finishes.
If you really wanted the paint to adhere, the finishes should be roughed up a little, like with a very coarse ScotchBrite pad.
When ready to paint, mask off the bolt, cover the scope lenses and tape any exposed turret markings.
A good primer is very important.
I helped a buddy Krylon paint his rifle and it came out pretty good for a garage job:Maryland Shooters - View Single Post - The Official Precision Bolt Gun Picture Thread