Posted for "richard  titsworth" <rtitsworth@mindspring.com>:

 Any pictures?
 
"""
 Recently, I had a machinist friend  look at the problem and he came up
 with a solution that seems to work pretty well.  He machined a dummy
 fuel cap (aluminum plug) that mates with the exact surface where the
 regular fuel cap seals.  I  lined the opening with a piece of vinyl tape
 and placed the plug into the opening.  Then with an exacto knife, I
 trimed the tape  where it  appeared above the plug.  BINGO,  I now had a
 perfect circle inside the fuel tank opening that shouldnt be painted.
 Then it was pretty easy to add masking tape below the  edge of the vinyl
 tape to cover the opening and when the wing was painted, I ended up with
 a nice looking result.
"""