What kind of bubbling are you getting? If it is the powder making "air like" bubbles on the finished product then it is because you are getting clumps of powder on your product so when it bakes it creates a pocket. This can happen if you are running too much PSI through your gun or your gun is getting clogged and then "puking" the powder out once it starts to flow causing the powder to again clump and land on your product.
exactly why i asked what kinda powder and what gun..harbor freight powder clumps a lot