I'm with the other guys on the thinned oil or enamal paints. BUT, before I do a wash, I give the model a coat of gloss.
Apply the first coat of gloss and allow it to dry. Apply your decals, use any setting solution you like, than apply another thin coat of gloss over the decals. Let dry.
Apply the wash over the glossed surface, this allows the wash to run into and around all those recessed ares ie. panel lines bolt heads etc., without the effects of friction or surface tension. The gloss coat alleviates surface tension for the wash. It goes on alot easier.
When the wash is done a dried, give the model a clear flat coat and proceed with your drybrushing.