Please do paint the inside. Ordinary laytex is just fine. When you are feeding you will accumulate some debris and perhaps some mold. Either can hold very tightly to the styro, as there ARE spaces between the 'balls' that make up styro. Paint will make a nice smooth surface to clean. No, it does not hurt the bees...remember, it is formulated to be save for toddlers who might nibble.
Lloyd Spear, Owner of Ross Rounds, Inc. Manufacturers of round section comb equipment, Sundance Pollen Traps, and full color custom Honey labels