Despite MichaelE's answer, I would pre-wet the ballast. Otherwise you end up with a hard crust over loose ballast, which isn't a good thing.
I use acrylic matte medium, diluted with 3 parts water and two drops of dishwashing liquid per cup of solution. For best results, allow the talc (added as a dulling agent) to settle out overnight and decant the solution.
Matte medium is much more expensive than glue, but it is more flexible when dry and resists discoloration better than glue, so it's worth it to me. I buy it at Michael's when the have 40-50% off one item coupons.
