'Adding a Wash' or 'Washes' are terms used for the technique of highlighting the detail of models by adding very thin paint around the detail.
Items used for washes
A "wash" is basically a mixture of very thin paint, I use good quality artist oils as the pigment is much finer and does not break up when thinned. I thin this using Humbrol Thinner as this is a very mild thinner and reduces the tendency to 'attack' the paint work when applying the wash, the mixture is usually about 80/20 thinner to paint, but this varies depending on the result required.
In most cases the wash is black, but if you are modelling a desert vehicle for example it would be better to use a brown colour.
A number of techniques have been espoused by different people for adding a wash from coating the whole model and then removing the wash from flat areas to just adding small amounts of wash around only the areas required. The rule here is 'what feels good, do it.' There is no "correct" way doing anything in modelling, what works for one may not work for another.

Before applying a wash it is best to have completed all the vehicle or aircraft painting, applied the decals/markings and given the model a final coat of clear Matt ( I have found that a wash is better applied to a Matt surface as the pigment doesn't break up as much and adheres better. The final Matt coat also protects the paint work from the rigours and applying the wash and drybrushing, allow at least 48 hours for this Matt coat to dry to ensure it is "dry".
The technique I use is to add only a very small amount of wash with an equally small brush (0 or 00) around the detail to be highlighted, capillary action will run the wash along engraved lines and around raised detail. Any excess wash is removed using either another small brush or cotton wool bud moistened with thinner to pick up the unwanted wash.

kit kit
Basic painting of Model Wash added to Model
It is best only to apply a wash to horizontal surfaces, this reduces the tendency for the wash to run to the lowest point, this means when applying the wash to the sides of vehicles or aircraft rest the model on it's side to apply the wash and until it is 'dry', 'dry' being when there is no visible thinner on the model. Obviously the paint isn't 'dry' in the true sense but you can turn the model over and apply the wash to the opposite side once all thinner has evaporated (no thinner, no runs). Remember if using oil paint to allow a good 24 hours for the wash to dry before proceeding. You may have to re-apply the wash to some areas to get the desired effect again remember to clean up excess wash with a small brush. I always apply a wash fairly heavily because the final "Drybrushing" technique will subdue the wash and 'blend' it in to the surrounding paint work.
Once the wash has been applied to your satisfaction and thoroughly dry you can then move onto the next part of "weathering", that is "Drybrushing". Experiment with different 'wash' techniques until you find the best method that you are happy with and gives the results you are after, as I said there is no 'correct' way.

By Terry Ashley