Teresa painted the inside of ours after I built it a couple of years ago. I think it makes it easier to clean. We change the pine shaving litter once every couple of weeks, turning it in every day. About every six months I'll haul the pressure washer down and use it to wash down the inside and outside. In the winter the Gals put out a lot of moisture at night when they're locked up so it can get quite groady inside with what I'd guess is mold. I use just enough pressure to not strip off the paint. She used primer and then painted the inside with an exterior semi-gloss paint she got at Lowes. It seems to be holding up pretty well.