Just as warning , face slapping is one of the things (like breath play ) people do and are not aware of the risks. Especially if you hit the face hard, you really should learn about the risks first and maybe visit a course about how and were you can hit to lower (not negate) the risks.
Hitting the face should be if at all the last area to learn about how to hit. After you have a lot experience.
Hitting to hard or the wrong place by accident can cause broken face bones, burst eardrums, eye injuries and even brain damage ...