Once the filler has kicked, you can go back and saturate the glass with a coat of unthickened epoxy. A chip brush works best for this. You might have problems if you use an epoxy formulation that blushes alot when curing... blushing is the formation of amine acids that float to the surface as epoxy cures, and it prevents subsequent bonding unless wiped off with water. I didn't have any blushing problems with the epoxy I used, which (for this job) I got from Raka Inc. I was VERY pleased with their products, prices, and service.