What is used to thin HXTAL so it can be used as a clear coat? Can it be used to fill pinholes on the surface of fused glass? if yes, thinned or full strength?

You can thin HXTAL with Acetone for clear coating surfaces. For filling holes, you'll want to warm up the HXTAL in a hot water bath to thin it down. Don't use the acetone as you'll need air movement around the epoxy in order for the acetone to evaporate out of the mixture in time.