I painted the inside of my 13" Coulter Dobsonian with Rust-Oleum latex brush-on chalkboard paint ($10 at HomeDepot; the two coats I applied used up about a quarter of the 30 ounce can), because I heard that chalkboard paint produces a nice and flat finish. By and large, it did. In some areas it wasn't quite as flat as ideally, but those were rougher areas, so the light will be scattered there anyway. It's the flat and smooth areas that are most important.
I will let it try for a while before putting the optics in, and then there'll be lots of moon out, so it'll be a while before I can seriously try it out.