A good start is to scan old paper, cardboard, wood and whater. Or photograph it. And use different blend modes in layers. Sometimes I make a layer beneath the main one, with a non-painted variant of the surface, and make scratches and other irragularities with different shaped brushed that do follow the actual shape of whatever you are making and make rust & dust in places where they appear. The ones brushes that come with PS are actually quite good for a job like that.