I've used a thin wash of black india ink to get a nice "weathered" effect on my ship models. It's inexpensive, pre-mixed, lasts forever and is easy to apply. The trick I've found is that by rubbing in small amounts at a time I can gradually build up to the desired darkness. It dries fairly quickly, but you can control the intensity after it dries by using small amounts of a product like "Goop" (the hand washing stuff).