I find that if you can shake or tap on your casting box for awhile, after pouring the cement, the air bubbles come to the top and will the help in a smoother finish. A man that works with cement said they use a vibrator to remove the air. I hope that this might help you .
Elane

You're using concrete which is portland cement, sand and gravel or are you using a mortar mix, portland cement and sand?
Some leaves create air bubbles and there's not getting around them. Make note which leaves have the air holes and see if it's the same ones all the time. You can fix the holes using a slurry (runny mix) of the same recipe that you used for the casting. Sometimes your mix is too wet; let it sit and dry out a little before applying. Some times you mix too much air into your mix when you're stirring. Just let it sit a bit before applying to your leaf. Billie