Bubbles in epoxy resin may be from one of several sources – air or gas escaping from a substrate (when used for flooring for instance), humidity in the air or choosing the wrong resin for the particular job at hand (they have varying thicknesses and properties). Working with the right resin (art, tabletop/floor or marine), in the right environment (70-75 degrees, less than 75% humidity) and ensuring that all materials are room temperature will help. When bubbles do occur – applying heat to the material before curing (very carefully) – with a heat gun, butane torch or barbecue lighter will vent (pop) those bubbles.
NOTE: To avoid pinholes in the finish when applying epoxy resin flooring over a porous slab – it is best to seal the floor first with primer. (And it may take two coats – but the process will be worth it!)