I think it's wise not to hit a child in hopes of teaching her not to hit.
My 17th month old is in the face-slapping stage also. Sometimes he'll do it when he's upset/angry, but more often than not he's just excited and hauls off and slaps me not realizing that it hurts. I've taught him to use gentle hands by showing him how to touch gently, so when he does slap, I quickly say, "You hit, you sit." I then sit him down on the floor, in a chair, wherever is most convenient, and I tell him to use gentle hands. When he touches my face or arm gently, I know he's ready to get up and play again.
Sometimes he gets really upset about the sitting part, but he's slowly starting to put two and two together. While he still slaps some, it's not every time I pick him up anymore, so we're making progress. It's been a couple of weeks, but like everything in a little one's life, it takes a lot of practice for them to know the rules and to understand the rules for hitting are the same no matter where they are. We have whole days go by without incident sometimes. Eventually he'll be on to learning something new, and this will just be a memory. HTH!
ETA: After reading some of the other responses, I have to say that the main reason your time-outs aren't working is b/c your child is too young for them, not b/c your parenting poorly. There are many tools that parents can use other than hitting a child, but the tools must be age-appropriate. Put the time-outs in your tool box for later on. :*)