A rough sand to achieve a mechanical bond is meant to be in the epoxy and as I said perhaps the glass is scratched. I have a similar effect happening at the bow and stern where I added a second layer of glass about an inch wide. I laid masking tape and after about 1-1 1/2 hours I neatly trimmed the overlapping glass but where the utility knife cut are the ends of the fibres which are now visible through the epoxy. For me it's not really big enough to worry about but I wish I knew what was happening there just for future reference.