I black lined my Tau by coating with a gloss coat, then using a thinned oil wash. Capillary Action is so cool!
In this image you can still see a few places that still need a light wipe with mineral spirits to finish cleaning the oil wash off of the high plates in the armor.
An acrylic wash would have stained the armor plates. With the gloss coat and oil wash, you can come back a week later, and it will still wipe off. When you have it just right you seal the whole thing with a matte finish to bring the shine back down.