A q-tip saturated in cooking oil or any other non-drying type oil will suffice too...Just put it on the areas that you don't want paint to stick...Using a q-tip allows you to paint the area with the oil well enough to protect it from getting it on it...and you can wipe it right off.....I have painted a bunch of rifles and scopes before...none of my Handi's though...and I never had any problems with them...
Mac
I did a similar thing on a steel scope I had years ago. The outside was quite rusty, but the optics were fine, so I sanded away the remaining bluing (and rust) and gave it a coat of flat black paint. I used a Q-tip with vaseline and smeared it on the lenses and blasted away... worked fine, and the vaseline cleaned up easily with clean rags dipped in Birchwood Casey degreaser.
Have used the same trick to fill oversized holes - like the one's in a pallet wood stock that sometimes get bigger because you take the butt plate off to get at the stock bolt and remove it from a frame just one too many times
- fill the screw holes with sawdust mixed with wood glue or epoxy, smear some vaseline on the back of the stock, and on the screws, then screw them into the holes. (I chose not to put the butt plate on at this stage - just in case.
Though if you are feeling confident you could smear grease/vaseline on the bottom of the stock, and butt plate and screw it all down until the sawdust mixtue sets.)
Wipe off any of the sawdust mixture that gets squeezed out after the screws are in, and wait. When the mixture has set (no harm in letting it go overnight), back out the screws and "voila" nice custom fitted screw holes. Important note... fill the holes with the sawdust mixture FIRST before even opening the jar of vaseline (or whatever grease you choose)! If you get ANY in a hole, the mixture will not hold and you will have just created
a new project! If that sounds like learning from experience... I'll let you make up your own mind