Surfboards take a lot of abuse from rocks, shells, transportation to the beach and pounding surf. Small dings should be expected, and are easily repaired. Cracks, however, require more care to fix because they can grow larger, resulting in the board taking on water internally, which will add weight and cause it to perform abnormally. Surfboards are made of fiberglass polyurethane or EPS (expanded polystyrene). Under the outer watertight shell is an internal core of foam. Some boards have wood laminate top surfaces, while others have a combination of fiberglass or EPS standing surfaces.
Inspect the insides of the board to make sure that it is dry, as the soggy foam of a board will never dry if you seal moisture in and the repair gel won't laminate properly. Press down on the board at the cracked area to see if water comes out. If your finger fits inside the crack, touch to see if it feels wet. Lift the board and assess if it weighs more than usual on the cracked end. Let the board dry for several days before proceeding if any of these checks indicate that the foam core may still be wet.
Lay the board in a way that gives you easy access to the cracked surface.
Sand the crack using your rough grit sandpaper until you can see the fiberglass cloth that is between the outer shell and inner board. Sand a larger area than the crack, because the crack may be linked to smaller cracks that are not readily visible to see.
Spread a large amount of your resin type repair substance liberally over the cracked area so that the entire crack is covered. The resin should be slightly mounded over the crack. Spread the substance around the edges of the crack, like a band aid, covering all of the sanded areas to ensure that there will be a tight seal and that you won't get any future leaks in the area of this repair. Smooth out the resin with your spatula or stick.
Let the board dry overnight. Test for dryness by pressing down on the resin substance to see if it is no longer tacky and has a solid feel.
Sand down ridges gently with rough grit sandpaper. Continue sanding with fine grit sandpaper until the resin is smooth and flat on the board.
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