A wash is a very light coat that is practicaly transparent. Testor's airbrush thinner works fine to "cut" or thin their paint. Regular thinner tends to make the wash sticky. However, enamels are tricky to use as washes because they etch into the topcoats and plastic. If you make a mistake, you may be stuck with it.
I like to use acrylics ( both model and artist brands) and inks (india is common) for washes because they are easily thinned with water or alcohol. Pastels too can be used when mixed and thinned with water and dishsoap and are nice for weathering because pastels will give the surface some dimension.
The key to a good wash in any medium is to have a very soft, pliable brush in varied widths and thicknesses. Make sure it is soft and of high quality. Use light brush strokes in one direction, and practice on scraps to get the feel for how much pressure you add and to see if you got the right paint to thinner mix.
A good brush and brush stroke will provide cover to highlight detailed areas without damaging fragile parts.
Keep researching FSM for the many applications and mediums for washes.
Scott