The photos look as if you just busted up the gel coat and a little of the laminate, but didn't crack the rudder so that water would penetrate inside. If water will penetrate to the interior (you will have to take a closer look), then you need to fiberglass up the damaged area with fiberglass cloth and resin. To do this, you need to grind out the area on both sides of the rudder, clean with acetone and then apply fiberglass resin, layer on cloth, wet down removing all the bubbles and keep applying cloth material for strength. The resin (polyester) needs to be cured with MEKP which will come with the resin. After it is cured, you sand it smooth (80 grit)and apply a polyester white putty (also cured with mekp, but it might be a solid toothpaste like consistency rather than clear water like consistency). When that is cured, you can sand it smooth again, this time getting the same shape as the rudder. When done, you apply gelcoat (white). Get gelcoat with wax. You also put in MEKP and thin it with acetone or Styrene Monomer. Spray it on with a Preval Sprayer (found in automotive stores or good sailboat shops/stores). When cured, sand it with wet/dry 320 grit sandpaper, then 400, then 600. When done sanding, buff out with a fiberglass rubbing compound for a shine. If your repair does not require fiberglass cloth, then just fill the voids with the fiberglass polyester putty and apply the gelcoat as mentioned above. If you want, you can use Marine Tex white, but that is an epoxy product and not a polyester product, so there is a compatibility problem with gelcoat. I do not recommend painting it. Gelcoat is what the manufacturer used and so stick with that. I do not recommend Rustoleum for fiberglass repair. I do not recommend paint for fiberglass repair unless you are painting the whole boat and then you wouldn't use Rustoleum. Use that for your wrought iron home products. LOL
