I have had good luck painting the outsides and edges with a good quality
white latex paint.  I leave the insides alone - the bees do a good job of
covering that surface with propolis.
 
Tom Fisher               | "The light that burns twice as bright
Purdue University        |  burns half as long....and you have
[log in to unmask] |  burned so very, very brightly, Roy."
                                                   - Eldon Tyrell