I used oil washes on my IF Land Raider Achilles. You can work with it and tweak it because it dries so slowly for an hour if you want.
Then clean up with a q-tip and mineral spirits. My method is just a tube of super cheap brown oil paint (burnt umber) then I mix it with mineral spirits, and weathering powders.
Just BE SURE to give the model a coat of gloss varnish first. This protects the model, and improves the flow of the wash. Give it plenty of time to dry before handling.
Once done hit it with matte varnish. You can mix different pigments for different effects IE soot around exhausts, rust/mud around tracks. A lighter dirt color on the hull around rivets, and to make rain streaks, etc.
Added a picture to show you how much control oil washes give you. You can see how little effect I went for around the rivets, and such. Enough to give them some definition without dirtying the model too much.