First I realy don't think you airbrushed on to much Future,

Have you ever tryed using 90% to 100% rubbing alcohol mixed with pastel chalks to make a new form of wash., or have you ever tryed a rustall kit. There's a small jar of black and white wash. That's what I used on my Merkava.
http://spruebender.net/forum2/index.php?topic=493.135Oh here's one more type of wash you can try. First you would have needed to paint the hole model with what enamel paint as it's base color. Then use dark of black acrylic colors and acrylic thinner to make a wash. This two is something I use on mini of my projects.
With black and some lighter colors of the pastel chalks mixed in for the right shade of dark or grit., the rubbing alcohol will help it dry faster. You must use a 0/10 thin paintbrush if you can't get it to run down and through the seams as you may want. An you may want to try using this same paintbrush with you wash and mineral spirits mix.