This house is used as a secondary dwelling by bats on their hunting grounds. Never use chemically treated wood, this would make bat males sterile. Paint only outdoors, with the following products: To protect wood from rain, don't forget to brush it with linseed oil, 2 or 3 times before fixing it. To keep the nest box warm enough, consider painting it with a walnut stain, which will darken it and better capture the heat.