It's thought to say without seeing the 'scratch' but if the fibers are exposed, you may need more extensive repairs such as sanding or grinding the area out and layering some glass cloth bedded in epoxy ( like the West Systems line of products). If the scratch is not too bad and you will be painting the area, Marine Tex will work. That stuff gets really hard and is tough to sand so get it on as even as possible. Read the directions of course for surface prep. The color probably won't match but that us not an issue if you are going to repaint.
If you blasting removed any gel coat, you should also barrier coat with something like Interlux Interprotect 2000 before bottom painting. If you use that coating, read and follow the directions exactly as specified.