First and foremost, if you care about the finished look of your SUP repair, smooth, watertight and color, pay to have it done by a professional. It you have experience or its a minor issue I have some tips from a repair professional. If the board is made from epoxy, use epoxy, not polyurethane fiberglass resin.The two resin types don't play well together! Dry the repair area completely, inside and out. If water remains in the repair area, the resin will create non stop air bubbles as it heats to cure, bubbles are never good on a repair.
Most important, mix the catalyst precisely with epoxy. It is a difficult medium to work with and if it does not dry completely and remains sticky, rip out the repair and do it again. Humidity can add to the challenge. Its also very important to stay with the repair until the epoxy sets up and gels ... you may work with it while its curing. Hope this helps! SunCure is another option for smaller repairs and it works well.