I begin mixing my washes, whether oil- (Testors, which I use most of the time) or water- (Vallejo Acrylics) based, by dropping a little paint into a container. I like to use the plastic cups that yogurt comes in; the plastic stands up even to lacquer thinner and the yogurt's good for you. There are dedicated hobby eyedroppers available in hobby shops for transferring the paint from the bottle to the cup, but if you know somebody in the medical profession, they can grab you a handful; some tool vendors at model shows also carry them in bulk. It's more economical to underestimate the amount of paint than otherwise, because I can't save washes - the solvent breaks the paint down and in a short time the pigments settle into clumps instead of being dispersed in a thin coat. I can always make up a fresh wash if I need more.
Then I add the solvent - mineral spirits for oil paints, water for the acrylics. Rubbing alcohol, as djnick suggested, helps the acrylic wash flow better. A drop or two of liquid detergent added to the water will do the same. How much solvent do I add? That depends on how opaque I want the wash to be. Another advantage of mixing washes in yougurt cups is, the plastic is usually white. So I can brush the wash up the sides of the cup and see how opaque it is, then adjust the density by adding more paint or solvent.
Different finishes will produce different effects. If you're building a vehicle, a gloss finish will cause the wash to settle into crevices like panel lines. After the wash has had a few minutes to set up it can be wiped off with a rag as ryoga suggested; wiping in the direction of the airflow, gravity, or whatever will help weather your model. If you're applying the wash to a flat surface, it will still settle into the crevices, but it will also stain the raise areas also. That's where you'll want to drybrush with the base and highlight colors to pick out the raised details. I prefer doing that as opposed to the wiping tecnique because wiping can rub off the underlying paint, plus the rag can absorb the wash out of the crevices.
I have not had ryoga's experiences with oil based and acrylic paints. When acrylics - Testors in particular - first came out, they didn't perform well over oil-based paints. Nowadays, I pretty much use oil-based and acrylic paints over or under each other with no ill effects. I can apply a wash made of the same kind of paint as the base coat as long as I allow that first color to dry thoroughly. The wash must be brushed on as quickly and with as little scrubbing as possible, or it will dissolve the base color and make a mess. I don't find that Dullcote makes an effective barrier against an oil-based wash; a light touch is the best protection against disturbing the base coat when working with oils over oils.
When the wash is thoroughly dry, I can drybrush over it to restore the base color to the raised areas. A lighter shade of the base color applied sparingly makes the higlights pop. I'll use oil-based or acrylic paints to achieve these effects.
It won't take you long to master wash and drybrush techniques, I promise; all you'll need is a little practice.
Last edited by Mark McGovern; 04-26-2010 at 10:56 AM.