When we got home from the movies tonight - saw This Means War which was hysterical - I got out my homemade chalk paint. Well, let's just say it was no longer the consistency of paint. My first thought was that I should use less Plaster of Paris. My second thought was that this recipe may require immediate use so mix only what you need when you are ready to paint.
I added quite a bit of water and painted a chair that I had in the garage. I noticed it getting thicker as I was painting. So...I'm still working on the perfect recipe. But, for now my advice would be to use less Plaster of Paris and don't mix it up until you are ready to paint. If it thickens as you paint just add a little water.
I'm determined to perfect this so I'll keep you posted!