I think this one is quite handy. As always we are using decals to help us on the little things which are hard to paint. In this case, we are talking blotches and spill and alike. This is one sheet in B5ish, containing over 70 artworks in various colors and sizes. We also would have mug stains and hand prints and more. It can be oil, and can be paint... it can be what ever you like it to be. As a little study, I have used it on the tank noozle of a 32nd scale WWI bird (Hansa).
Seal it nicely with clear. Allow some of it to build up to entirely blend in the decal. It doesnt have a specific scale but it generally would work best in 48th and larger. You can use it in the 72nd scale as well, but in this scale we would have stains the size of a jar of paint, rather than those of a mug. Just give it a shot. There is surely something in it for you.

Ouh, and btw: The "marbling" effect you see along the rib tapes has been achieved using trusty our trusty trinity stencils. Just saying....