I didn't realise that I was using washes the wrong way until I came across a You Tube video. When I tried this method it all made sense, used a lot less wash, and gave the effect that I always saw on the well finished models. It is so simple I kicked myself.
All I do is work on a small area at a time, get a thick brush, and liberally brush plain water over it. Then get a very small pointed brush, dip it in the wash, and then dip it onto the water soaked surface. It simply disperses it every nook and cranny, and sinks to the surface of the plastic. It dries very quickly.
This way it accumulates naturally where it would on the real thing, and feathers the edges so that there is no hard line. It works like a charm.
For large flat surfaces, just wet the surface, put a few splodges of wash on, and use a thick soft brush to spread it around, don't worry about brush marks, they will all vanish by the time it dries.