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Mix. You want to make sure that you aren't adding air to your plaster, or you will get bubbles which will cause problems later. So if you use an electric mixer make sure to keep the blade deep in the plaster. Or, just take your hand (a glove is useful) and place it at the bottom of the bucket, and slowly move your hand back and forth across the bottom of the bucket. This slowly wets all the particles. When you can draw a line in the plaster and it doesn't immediately flatten back out, the plaster is ready to pour.