If you read the posts that Beth so nicely collected and posted, you will see a discussion at great length in one thread on to paint or not to paint the inside of the boxes. Some were concerned about the difference in moisture content if you paint the outside and not the inside of the wood. Keep in mind, one of the first things the girls do when setting up house is to propolise the entire inside of the wooden box to clean and sterilize it. Propolis is the gummy, sticky resin of trees and makes a very good sealer (it seems to work well for the trees). So don't paint the inside of the boxes.