Mixing at too high a speed.
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This can be trapped air and needs to be worked out. This can happen if another background coat is used for a repair.
These are caused by air caught in the plaster or skim or if walls are dried out too quickly or plaster was too wet when put on wall. To get rid of them sand down the bubbles and then use filler to fill in indents and sand down again
Bubble are courses by many reasons but first most common diy work is carried out to fast need to be sanded to a level surface ..
A chemical reaction can cause this is so it my be loss as well as bubbles remove loss and bubbles and start again but contact a professional