Your board is almost finished. But you now need to install a leash plug and the sand the board smooth.
About 6 to 8 cm from the tail and close to the stringer, use the electric drill with 3.4cm diameter drill-bit to make a hole for your leash plug. Be sure the area is deep enough before you start drilling. With a small knife, peel off the fiberglass over the hole and remove as much foam until the leash plug can sit flush with the deck.
Prepare 100ml of hot coat resin with catalyst in a small paper cup. Pour a small amount into the hole. Then place the leash plug into the hole. Add more resin and push the plug into place. Use a used paint brush to wipe away excess resin. Let it cure until hard.
Put on your surgeon’s mask and safety glasses and plug in your electric sander. Start sanding the bottom first. Sand everything until the gloss is gone and all the bumps and lumps are gone. Some areas you will have to sand by hand. Be careful not to sand too much or you’ll damage your board. If you do damage anything, just repair it with a bit of resin and fiberglass cloth if needed and then sand it smooth. Using a fine grit sand paper works best but takes a longer time. Using a coarse grit sand paper may be faster but very easily can damage your board. Using the fine grit sand paper will make your board look better so it’s recommended.
When you finish sanding everything smooth, get a bucket of water or, better yet, a hose with running water. Using the wet-sand sand paper, and in a circular motion, sand every area of the board until it becomes a little glossy. This step is very important because without this step, your skin will be irritated and itchy after touching your surfboard.
That’s it! Your board is finished. To strengthen the board, don’t use it for another 3 days. This will allow all of the resin to properly cure completely.
If your board doesn’t look like the ones you see at the surf shop, don’t worry. Even professional shapers have made a few bad surfboards when they first started. After you make about 3 or 4 boards, you’ll become comfortable with the process and it will get easier.