One of our club members (Gary Nichols) uses an interesting technique to weather his buildings. He uses liquid shoepolish to go over the building to bring our detail. It is water based and can be washed off orv wiped. Since it is thin to start with, it makes an excellent wash. Most come in applicator bottles with a sponge tip so working with it is easy. Those that went to San Diego might have seen his clinic on plastic building construction and weathering.