Another diluting agent is Gel Medium. It looks like white milk but is thick like syrup. Buy it at an art store. I asked a clerk for "something to add to my paints for it self-levels" and she handed me a $5 jar.
Using a dimpled pallete, I make a pool of Gel Medium in the bottom of the dimple. Then I dab the skin tones around the rim, then slowly stir in the colors I need. (These are with acrylic paints.) If you keep the paints good and "wet", it should dry fairly smooth.
Any brush painting will leave brush strokes. (It's one way they identify Renaissance artists, by microscoping the brush strokes and comparing to other works.) If you want REALLY smooth, you'll need to learn how to airbrush.