Im going to be using 2 part epoxy for filling in sunken inserts - the stuff they put scorpions in etc and then sell.
Its self levelling, can be polished etc.
The test on filling with the glass coat was VERY disappointing - basically, this stuff is crap for what i want it for. it does dry hard though and would be good for protecting drop targets if painted on as i found no reaction at all to 3 different paint types - acrylic, enamel and soft enamel.
The insert i filled created its own air bubbles - they werent there when i applied it but appeared as the glass coat cured.