once the wash is on.and the thinners have dried.Use a cotton cloth very slightly damp with thinners and wipe off the majority of the wash leaving the wash in the recess's.
Hope this helps,others probably know better than me.
To my mind, that's much too dark Dave. German Grey was very dark in reality, but I always think it needs to be lighter to allow for scale.
Also, a sludge wash is heavier than a standard wash. I add talc to mine to give it ' body ' & use it for noticeable build-ups of dust & dirt. I wouldn't use it all over.