when you do washes, thin them out. 3:1 or more. This will create just a film that will settle away from the ridges and into the recesses. The burnt umber would have worked well as a thin wash. Oil washes work very well on acrylics. The acrylic resists the solvency of the turpentine/turpenoid/mineral spirits you use to thin the wash. If you are going to use an acylic wash, let the acylic base coat dry and cure first. ( at least 24 hours is what I do) Acylics, if water based, can be thinned with water. Tap or distilled. (personal preferences for some) or you can use the commercial availiable thinners for the brand of paint you are using. Or alcohol. Same effect. Once the wash has dried and set do your highlights. If you are going to paint your shadows and highlights rather than use washes you will have to take more attention to technique. And layer rather than blend paint (like in oils).
This is a pretty big subject, and whole books have been devoted to just this topic. I suggest Kalmbach publishings book Painting Realistic Military Miniatures (available through this website.....imagine that....) and Shep Paines book on dioramas. (available same place) worth every penny and will certainly give you a good oversight of the concepts and techniques.
My other suggestion would be to use the search function of the forums and do some surfing in past posts. Use every keyword you can think of in the search and you will find loads of info.
It would probobly help a little more too, if you were a little more specific regarding your questions. As I said its a big topic. Patience and perciverance will pay off, and as always practice.
Mike
Mike
"Imagination is the dye that colors our lives"
Marcus Aurellius
A good friend will come and bail you out of jail...but, a true friend will be sitting next to you saying, "Damn...that was fun!"