Great resource for weathering
I really like the Vallejo pigments for weathering my model trains and structures. They are very fine and easy to apply by using a stiff brush to rub them onto the model. Although I normally apply mine dry, they can be mixed with varying amounts of water and applied like paint. If your model will be handled a lot, you may want to apply a clear top coat such as Dullcote. The Dullcote will cause some of the pigment to disappear, so you may find that you need another layer.
