This article is from the Miniatures Painting FAQ, by Brenda Klein firstname.lastname@example.org with numerous contributions by others.
It seems that once in a while, even though the inks and
washes have been mixed properly, they end up drying, not
in the low spots like they should, but on the high contours.
It has something to do with the density of the wash and
the slickness of the surface; on matte surface the effect
is more prominent than on glossy surfaces. It happens
because a pool of wash in a recess starts to dry from the
edges, then the rest of the paint in the wash adheres to
the already dry paint, producing a ring of paint around the
recess. There are four methods that can help solve the
1) Add a small amount of rubbing alcohol to the wash.
It lowers the surface tension, and dries faster. This
may be a drawback for some painters. Some model
railroaders have been doing this for a while now.
(Thanks to Coyt D Watters for this tip.)
2) Add a little dishwashing detergent to the wash. It
helps the wash stick better. (Coyt again...)
3) Use small amounts of wash, allowing each to dry
before applying the next. Blow gently on the wash
after applying, from the top, to keep the pools
in the recesses where they belong. If the wash is
thin enough, it'll dry with a minimum of blowing.
4) Mix a new wash, thicker. It might work better, being
thick enough to keep from creeping, or maybe with
just little different density.