Charlie:

I recently ran into the same problem painting my wings.  In the first wing, I installed an old cap and just painted over it.  .  In the 2nd wing, I very carefully used masking tape to seal the opening.  In both cases, I had a problem in that the paint interfered with the sealing surface of the fuel caps.  SOooo, I did it over by sealing each opening with masking tape (not an easy job), then I sanded eack mateing surface clean of paint, then did a better job of masking the mateing surface of the opening as well as the opening itself.  Hope that makes sense.

Good luck - Bill Hogarty