Bill, when using any sort of wash make sure you give your model a couple of good coats of a clear gloss coat (such as Future floor polish) and let that cure for at least a day before applying the wash. The Future will protect your lovely paint job from the weathering you about to do as well as helping it to flow a lot better.
OK, your questions how soon can you apply it after painting, well depending on the weather conditions I would give it at least a couple of days before applying the clear coat. Turpenoid makes excellent thinner for artists oils, just mix it in a pallet to about the consistency of coffee.
Cheers
Darren