I use artists water colour, the stuff that comes in the tubes, not the blocks, and mix it with a little water in a palette. I then paint this along all the panel lines, as well as any crevices on the model, and allow to dry.
As per Jeff's post the best way to remove it, is with a wad of kitchen towel in a small V shape, I hold mine in a pair of self locking tweezers, lightly wetted. Always wipe off at 90 degrees to the panel line, if you wipe along you will remove the wash from the panel line.
It can be a fairly slow process applying the wash this way, but the effect is more than worth it. Also I have found that a thicker consitency works better over finely engraved lines where it is less likely to be pulled up by the removal process.
Hope this helps.
Karl