The house I'm working on is very old. Could that be the reason for air bubbles going over plaster? How do I get rid of them?
It's difficult to say what you may be doing wrong without actually watching you. You may be over aerating the mix; keep the paddle speed at moderate to low & use a circular motion, around the tub, rather than up & down as this tends to draw in more air. The mix may be slightly too thick & this tends to trap air & makes it more difficult to dispel as you float on the first coat; it also makes it over thick! Your trowel action (both angle & speed) could also be a contributory factor. You've obviously mastered the basics; try practicing different techniques on a gash bit of board (or wall & scrape it off before it sets) to see how you can improve your technique.
Search the TrustedPros directory and discover the best contractors in your area.
Find your home service pro