This is a terminology and perhaps application misunderstanding. Washes are highly thinned paints applied after you're finished painting. What you're attempting to do is preshade. Typically, preshading is done with a much darker color, i.e. black or burnt umber. It can be applyed with either brush or airbrush along the panel lines. You don't have to be precise with preshading, in fact it's better that it's not precise.
Preshading is tricky because you can easily cover it up when applying the paint coats. You'll have to apply OD and Neutral Gray in multiple very thin coats, almost misting it on. You want the preshade to be slightly visible through the top coats.
A wash is a highly thinned oil or acrylic paint that is applied selectively to panel lines or wheel wells, etc. to pop out the detail. A good reference site is swannysmodels.com for tips on washes and such.