There is a product that is widely used out here on the West coast on boats and surfboard repair, made by Evercoat, called Formula 27. It is water resistant and fiberglass bonds to it super. The material is great to work with. I have been using it. It is an alternative to Bondo. After it goes off, when you scrape if off the cement floor it pulls the cement off!
This info says it's white, but it's gray if used without a coloring agent. Since I will be painting
my egg, I skip the color at this point. Evercoat makes many resin and gelcoat products.
Formula 27 All-Purpose Filler
is perfect for permanent repairs of fiberglass, wood, metal, cement, stucco, and drywall. It is the finest quality, extra-white plastic filler for filling dents, scrapes, and gouges. A true "miracle mender", it won't crack, pop out, or shrink after curing. A long time favorite of boaters everywhere, Formula 27 can be mixed with Evercoat Color Agents for color matching. White cream hardener included. 1/2 pint