My first question is: what is the wash intended to do? If you want it to shade the model, then you need to mix one that’s clearly darker than the base colour. OTOH, if you intend it as weathering, so to represent dust and dirt, pick a colour that represents that.
You say you made a wash of burnt umber with water. That sounds like artist’s acrylic paint, the kind that comes in a tube? I haven’t used that in a long time, but for a wash made from acrylic model paint plus water, I liberally apply it all over the model and don’t wipe it off at all except where it pools (usually at the bottoms of vertical or strongly sloped surfaces). The trick, though, is knowing how far to thin it. This is a bit of an art, but luckily, every AFV model gives you a perfect place to test it:
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Well, unless you want to enter your model in an IPMS competition, anyway:
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