:-) I feel for you, I've been there.
Basically you need to protect your paint and decals from the affects of the thinner used in the wash. If you have used enamel paints, or enamel clear coat, then the thinner used with oil paints will attack it. It is also possible that it can attack acrylic paint/clearcoat. I use a wash made from artist oil paint and white spirit. To protect the paint and provide a clear coat I hand brush on 2, thin, coats of Klear (Future in the US) acrylic floor polish, apply the decals, then add 2 more coats of Klear, to further protect the paint and the decals. After washing I add a matt coat, enamel or acrylic. HTH
You may want to have a look at my article about hand brushing Klear.
The Future is Klear Mal