You can do whatever works better for you. You will get much heavier coverage that way. An alternative is to use mineral foundation alone on damp skin, which gives somewhat more opaque coverage & keeps foundation from looking caked on. Using a powder foundation over a liquid foundation can often result in a caked on look, although misting thoroughly after application can really help. Mist once lightly, let it dry, then mist again, let dry. Just don't soak your face all at once or it can ruin your foundation's finish. A couple of light mistings is better than one heavy misting. You can use plain water, water mixed with a little glycerine or a finishing spray like Skindinavia that's made specifically for this use. I've tried all of these types of mists & haven't really noticed a big difference between a bottle of water and a $30 product, although I do mix together my own ingredients for this purpose that I've been happier with the results overall.