Its not really difficult. I watched vids by a guy called "night shift models" and he shows the basic ways, then one just needs to bite the bullet and go for it.
I think the way to do it is to try work out what the look is you want to get. If it was a freshly applied wash ,it would be allot whiter and more uniform over the model than what I wanted to do. I wanted a late winter,early spring effect and therefore I did a wash and chipped it in high trafic areas.
Then I did another light translucent wash to show where the original wash might have been re applied but with some of the under-lying dark areas still showing through. I also applied a slight wash on some pannels like on the engine bay just to alter the look and give a little tonal difference to the white. If it is acurate I dont know but I like the look.
Then again chipped the high trafic areas but added some dirt/mud as the crew walks there.
The mud on the lower hull is more of a runny "slush" than heavy packed on mud, I reconed that the tank may have been going through some puddles where the snow has melted LOL
The problem with the wash is that one looses almost all the bottom tank colours. where thay do show through it is hard to tell if the tank is a mono color or camo bit then I guess this was the whole idea in the first place.
Theuns