First make sure that you can't just wet sand it (400, 600, 1000 and then polish). 1/16" sounds deep but maybe you are overestimating. Definitely, don't use bondo, it absorbs water because it has micro baloons and also is red and difficult to white color over; it is nothing like gelcoat. Don't use epoxy because polyester gel coat will not stick to it; nothing polyester will either. Buy some white gelcoat, with patchaid is good, or get wax (sometimes they call it exterior gelcoat and buy it at the place the pros buy). Buy some cab o sil (ground glass), don't use micro ballons, they are soft and absorb water. Get some mustard gel coat tint if needed. Use a toothpick to put a little, very little pin ***** of tint to match the boats gelcoat, usually a little bit beige; dab a bit on the surface to see. Then add cab o sil to the matched gel coat and make a paste that can be squeegeed to fill the crack (sand the crack first). Catalyze with mekp, you might go a little heavy if no patchaid. Overfill the scratch. Then sand smooth: 150, 220, 400 wet or dry, 600 wet or dry and 1000 wet or dry, careful to sand the sanding paper scratches well; you might be able to skip the 150 and 1000. Buff with polishing compound. If you are careful it will be unnoticeable, use at least the 20 foot rule looking at it. You may have to fill it a few times because of air bubbles. Wipe always everything in between with acetone. If that does not look good, then after filling you will have to sand a larger spot and mask the edges off with wide blue tape turned over on itself to form fine edge for spraying. Spray with gel coat. If it has patchaid it will set up quickly. If it doesn't then use wax mixed with the gelcoat to help it set up. After a little practice you will get the hang of it. It takes time, there are no short cuts or magic elixirs here. You are basically replacing what you scraped off.
