Treehugger
My impression of this stuff is so so, but I am no expert. I think I have learned, that it looks the best, if only applying it once, else it might end up looking ugly. Adding the stuff more than once, afaik dissolves the existing enamel paint (the wash).

Steffen Arndt
I am using (mostly) acrylics for painting thus enamel washes are no problem. Normally I am not using different hues of washes over each other. If I want to I am mixing it on the model. BTW I am talking about wash for a cockpit or a similar small area on the model. For the whole model I prefer a sludge wash with water paint umber and black...