Try wet sanding the area with a very fine wet paper like 600 grit. Sometimes as you smooth out the fractured glass strands it may wet out. Sometimes it works. You will know because the water of the wet sanding will just make the white disappear as you sand. If it works, you would wet it out once dry with epoxy and sand and varnish. If the fractured glass is too deep, It won't work. That white you're seeing is glass fibers. If you're thinking about sanding anyhow, give it a try before you pull out the Ros with 120 or whatever.