I painted 3 of the interior walls with a semi-gloss latex paint and used a single 4x8 panel of smooth Fiberglass Reinforced Board (cut to size) behind the roosts. I anchored the FRB to furring strips with flat aluminum stock to avoid adhesive fumes in the coop and sanded all exposed edges with an orbital sander. It looks good and washes far easier than paint. Although it's rather expensive (about $65/sheet) it should outlast me and the chickens.
I second that if you can afford it FRP is the way to go.
Scott