Flip and fire
Pot and mesh melts are full of tiny bubbles. These migrate to the surface each time the piece is fired. One way to overcome this is to do a firing with the (final) side down to the shelf. Take it to a fuse temperature. Cool and clean. Flip it over and do the fire polish for the final top side, This will reduce the number of pin holes in the top surface, even if it does not totally eliminate them.