If you are getting bubbles in each coat then most probably you have shaken the can improperly. Most brands put this instruction on the label- not to shake the can. They suggest you stir the finish. Another reason you can get bubbles is if you don’t put your brush in the mineral spirit before you dip it into the poly. Sometimes, you can also get bubbles if the material is thinner. Like, if you put water in the water-based poly which is already thinner.