I have tried weathering with water based poster paint, mainly wagons and coaches. I use the same method as wet weathering with Humbrol etc. i.e. mix the paint, dip in the brush, wipe brush on a piece of card, wipe brush on a piece of cloth or towel and then off you go with light brush strokes.. Try not to put too much on at once, and if you run a drop of water from your brush downwards it looks fine. The ony problem is when you pick them up it can rub off a bit. I have weathered a Black 5 with it and altho' not very good at least it will wash off. I think trial and error will improve results.