Make sure that any anti corrosion coatings have been removed from the brass stock, I normally use a very fine steel wool to remove the coatings, this will also burnish the surface of the brass for coating with the paint, I use a flat black stove paint, coat one side and bake in the oven for about 20 minutes at 225 degree's, let is set over night, then coat the other side and repeat the baking process, this normally gives a pretty good durable coat of black paint.

Good luck