Gentlemen,
In OGR, run 275 of last January, 2015 Tom Yorke did an article that featured many fine step by step photographs of a tiny store called "Acrylic Reality". This article detailed everything from undercoating, paint consistency,procedures and so on for using acrylic paints to transform a resin model (plastic) into a realistic rendition featuring brick, concrete, wood, metal and roofing into a realistic rendition of an old, worn building. Please look into this fine article and I'm sure it will answer many of your painting and finishing questions.