Scale models are intended to portray the original version of a subject, an aircraft, actually. Rarely will you see a real plane without chipped paint, or stains from exhaust or fuel spills. Models can be finished to exhibit the wear and tear of the real thing. The process of making a model look old and used is called weathering. The most common methods of weathering a completed kit are dry brushing and washes, two easy painting techniques.
Both of these techniques are used after the piece or model airplane has been painted and had decals applied. It is also a good idea to consider the paint types used. Use an acrylic or water based paint for weathering. If the finished model aicraft was painted in a water based paint already, make sure it is well cured before weathering. An enamel or solvent based paint used to finish the model kit will not be harmed by using the water based paint in weathering.