what Moses said, but not explicitly,
is that the paint type of your wash should be DIFFERENT than your base.
so he paints in acrylic an washes w/ oils.
do not try to wash w/ the same type of paint as the base coat unless you shoot the kit w/ future or some kind of gloss / protector coat first.
otherwise the wash, being 95% thinner will remove the paint like nobody's business.
i proved this on a airplane this summer!!
like dj, i use the watercolors now.
i thin w/ tap water and a bit of dish soap.
it gives me no trouble over Tamiya acrylics.
if you are not sure, and even if you are,
test it on scrap styrene or a test body you have laying around.
ed.