All my grey wagons, including the Bulleid opens are painted with grey primer from rattle cans. The Bulleids then recieve washes of very diluted black poster paint with a few drops of dish washing detergen sprayed on with a spray bottle. The wash should look like very dirty water. A paint brush is used work the wash into corners and crevices. This process is repeated allowing wash to dry between applications. I have found that it is best to place the wagon upside-down while drying to avoid unrealistic water marks in the interior caused by water pooling.
Once the desired effect is achieved and the wagon is totally dry it is dry brushed with white paint. Using prototype photos as a guide the axel boxes get a wash of black paint to represent oil stains and rust applied using weathering chalk, again consult prototype photos as a guide.