Air bubbles can form for a number of reasons. You didn't mention when the bubbles are forming, or the type of sprayer you're spraying with. Small bubbles form right away if you spray too thin of a coating. This happens more on the second coat for some reason. The enamels thick and won't layout without the right thickness applied. Bubbles can also form if there's air inside your spray line, assuming you're using an airless sprayer. Another possible reason is contamination inside the spray line, or on the surface, you're spraying. Make sure you've flushed out the sprayer really good with clean water before spraying the enamel. Make sure the surface is clean too. Spraying too thick of a coating can also cause problems. When the paint's sprayed on too heavy, the surface starts to skim over before the paint deeper below has dried, so it starts off-gassing through the top, creating air bubbles. It might be too dry in the room too.