if you have an art supply store near you you can buy either oil or watercolor burnt umber and use that. That's what I use and it works great. Simply clear coat your model (Future floor polish in my case) then use the burnt umber like you would any other wash.
Here's an article on it, lots of good tips all over this site and I recommend looking at all of them!
http://www.swannysmodels.com/Tools.html
"The basics of weathering" covers washes with oils and acrylics...