Washing without changing color of flat surface?

What are you using for your washes? Water-based or oil-based washes?

I use artist oils thinned with odorless thinner for my washes. What I do is load a round brush with the wash and touch the tip to the recessess and raised details. Capillary action draws the wash around where I need it to go. I then go back with a brush damp with the oderless spirit and clean up the spots where I touched the brush to the model, as there is usually excess paint at the touch point.

You could do something similar with your hard edges. Just go back with a damp brush and clean up your hard edges. Also, you can pre-wet the flat areas with a bit of odorless spirit before you apply the wash. Don't flood the model with spirit, just make the surface wet. This makes it easier to clean up the hard edges.

Great answer! I use water based... Vallejo washes. If I mix my own then it is tamiya paint and x20a thinner. But your prewetting suggestion was a great idea. Thanks so much for the reply.

An advantage of dry-brushing rather than washes (other than pin washes) is that dry brushing gives you great control over exactly where the color changes occur.  I use washes where there is a lot of detail distributed over and area where washes could nestle down and affect colors, but for flat panels with little detail, I prefer dry brushing to alter only areas of panel.  Or, pre-shading also works for those areas.

If I'm understanding what you are trying to accomplish is to have the wash settle on the sharp details such as corners, louvers, bolts, panel or hatch lines? If so then you have to gloss up the entire model so the wash just settles on those areas and is not absorbed by the entire surface area as it will happen over flat paint that acts as a sponge. 

I rather make my own washes out of turp and artist oil paints which flow much better than acrylic washes. 

I use Turpenoid and oils very diluted. You can also get good at blotting off the wash from the flat areas with tissue or paper towel. That creates an effect of its own.