Ken's making a good point about masking. Usually builders decide they want to fill some gaps after the sanding because it is now very noticeable to them. If you mix a dark filler and contaminate and stain the the strips next to the gap, you will create more work then you want. You will now have to sand it clean and if you don't you can end up with an ugly blotch when you finally wet out your glass. And that will look far worse than any gap. Small gaps get filled with the epoxy when you wet out your glass. So if it's really big you can fill it but to go around and fill micro gaps is counter productive.