The starting point for weathering is to apply a wash to highlight the groves between planks, the gaps around doors and so on. Both enamel and acrylic washes are available; my preference is for enamel based washes.
To apply the chosen wash, shake the wash bottle well; I actually prefer to take my electric stirrer to bottle of wash to make sure it is properly and thoroughly mixed.
As I prefer to use enamel washes, I have a glass dish containing white spirit handy. Take the Rigger brush and soak bristles in spirit, now dip it in wash and apply drops to side of coach or wagon so that the wash is drawn into the features to be highlighted.
The wash used for this job was A.MIG 1002 (Track Wash from the Depot Weathering Set).
What you will see is the wash run into the groves you want to highlight and there will be a small blob where you applied the wash. The excess spirit in bristles helps the wash to flow into plank lines and around rivets &c.
The models are left for 20 – 30 minutes for the wash to begin to harden and the Rigger is again dipped into white spirit. The excess spirit is coaxed off the bristles and back into the container and the brush is then laid on the blobs. Almost as if by magic, the blobs of dark colour are literally “drawn off” the side. If you look at the sides of the BG you will see that I have also applied the dark was over the door stops and they are now very pronounced, this effect can be “moderated”, “modulated” or in plain speak toned down in the same way that the blobs were drawn off the sides. If it gets a bit too wet and sloppy or you don’t like the effect you have created just leave it for another 30 minutes to dry and then start again. It is this ability to “rework” that always draws me towards enamels. If we were doing this with acrylic washes every line would have to be filled and then cleaned up immediately; using enamels allows you to do the whole model and then take a step back to view the overall effect, and then adjust the bits you think might not look “just right” to your eye.
When you are completely satisfied you can either leave it to completely harden (I reckon 5 or 6 hours, some say up to 12 hours) and then think about dusting on weathering powders, or you can move straight on to airbrushing a dirt paint layer on to your model.