To keep it somewhat short and simple; (I'll probably fail though!)
Dry-brushing and washes are techniques used to highlight detail and/or add a bit of depth and realism to a model. Washes generally are used to create or enhance shadows, with a follow up of dry-brushing adding the look of light reflecting off of highpoints.
The term dry-brushing is rather self-explanatory. The color used generally should be a lighter variation of the base color already on the model. (Experiment with different colors!) Get some paint on a brush and wipe it almost clean using a rag or paper towel, old T-shirt, new living room drapes, carpet...
There should be a small amount of paint left on the brush. Gently whisk the brush over prominent details, just touching the highpoints. After awhile you'll see that these details will start to stand out even more prominently.
A wash is basically paint thinned to the extreme. A whole bunch of thinner and a very small amount of paint constitutes most washes, but it depends on the look you're going for, so experiment with thinning ratios. Also experiment with different colors, but you generally will want to keep them dark.
To apply a wash, simply dip a brush into the mixture and apply to the model in areas of concentrated detail. It'll flow into the crevices and corners and create a fidelity of detail that, especially when combined with dry-brushing, will lend a realistic look to the model.
WARNING!
Be sure that the paint and thinner you use will not lift the underlying paint. ie; don't use an enamel wash on an enamel paintjob. There are many different types of paint and thinners you can use, experiment and see what suits you. Some of the more popular washes are made with oil paints and enamels. Also, some like to use India ink. Others use tempera paint. Again, lotsa choices. I prefer enamel washes (over a lacquer-sealed enamel paint-job) and watercolor washes.
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