I realized no matter how much I stirred - the lumps were NOT going away. In fact, I feel they became worse the longer I mixed. No worries! When I used the sponge roller to apply the homemade chalkboard paint, I just kept rolling over the lumps and they evened out perfectly. FYI - I used leftover latex off-white and black paint to make my homemade chalkboard paint. I don't think it matters too much what paint you use.
This truly was so much fun, and this paint makes a quality chalkboard surface. When you realize you can make a chalkboard in any color - well, who wouldn't want to try this?