Make sure you prime the walls or wash them. I prime with the cheap Walmart grey paint for about 1 dollar a can. Then paint from there. Or Rattle Can the walls the color you want.
They are excellent buildings and I have scratched built those buildings for many uses!
with the brick, I rattle can the red I want, then 'wash' the walls with the mortar colour I want.. either a light grey or tan and that fills in the mortar lines.. let it dry, then spray with a clear matt or dull coat.. then if there are other feathers like the windows sills etc, I paint them with an acrylic suggested above and off I go. Dan
