Check out my results from this post:
http://www.therpf.com/showthread.php?t=266682
I attribute my successful experiment partly to the Krylon 'glossy' spray which I applied both before and after the washes. Because it dried semi-matte, my wash tended to 'stain' as much as it 'flowed' around the surface details.
Of course you are working in much smaller scale. Oil based washes are very good but some thinners can be rough on the plastic, so use something less "hot" than turpentine such as the 'odorless' thinners made for oil painting work. Also, acrylic paints when greatly thinned really need a
surfactant to lessen the surface tension of the water (hence, the recommended dishwashing soap...) The key characteristics of a wash are flow and pigment density. Flow is controlled by thinning and pigment density is a feature of the source paint. Oils have a very fine pigment in suspension and so make very good washes. Acrylic pigments vary more and are usually of a larger 'grain' though hobby paints tend to push that as fine as possible (even bargain paints like the Apple Barrel brand craft paints I used on my project..) One advantage of acrylics is that they can be washed off (if not allowed to 'dry' too long) if they don't look 'good'.
The effect on a matte or glossy finish is important since in general a matte or semi-gloss finish will allow for a 'staining' effect while a glossy finish provides less staining and more flow for the wash.
When you start working with washes, decide exactly what effect you are trying to achieve. Washes can simulate dirt or grime buildup, general surface discoloration, shadow and reflectance effects, corrosion stains or other types of material discolorations. Sometimes it is purely for artistic effect by emphasizing the tiny details with contrasting color 'outlines'. So I recommend taking a moment to imagine how you want the wash to look, then adjust the variables of pigment color, viscousity (flow) and pigment density (opaqueness) to obtain the desired result. Subtle and harsh effects are both possible but it all enhances the model's scale appearance by simulating a more complex visual appearance - which is a feature of 'full sized' objects that capture and reflect light in a myriad of ways.
Hope this helps!
R/ Robert