Surfing School - Glassing - The Fill Coat
After the board has been hot coated and a fin box or fin has been installed, you are ready to sand. There are several methods of sanding depending on the sander's experience and equipment. This author prefers a fast disk sander with a 7 inch soft pad for quick results. However, considerable experience (and concentration) is necessary with this equipment as is very easy to sand through the fiberglass outer layer and ruin your glass job. Inexperienced sanders should start with a heavy, slower type grinder such as those used in auto body work, or even a drill with a 5 inch soft pad. These take longer but the risks involved are considerably lessened.
Sanding pads are soft or hard foam depending on application. Soft, 1/2" to 1" thick pads are used on rails and in dips or other non-uniform glassed locations. Hard foam pads are used when sanding the flat surfaces on decks or bottoms, and also when sanding fins. A typical solid backed foam pad is shown in Figure 01. Note that the base of a mounted fin is non-uniform and requires a soft pad. Inexperienced sanders should use the harder pad exclusively and sand the dips by hand. Eighty and 120 grit sanding discs are the most common used.
Professional sanding racks are very similar to shaping racks, and in non-commercial applications are often the same. The lining covering the sanding rack should be sandpaper rather than foam so the board will not slip when pressure is applied. In commercial applications, the two supports are closer together than shaping racks, and are placed in a special room used exclusively for sanding. The racks are also sometimes equipped with suction cups to hold the board very secure, and the room has a large blower for ventilation. It is essential that the sander wear a protective dust mask, and should also wear heavy clothing while sanding.
In the non-commercial, backyard environment, do the sanding outside in a shaded area on a cool day with only a slight breeze. Indirect natural lighting and outside ventilation probably is acceptable provided that only an occasional personal board is to be sanded.
To sand a board, lay it flat on the racks, deck up, and using an 80 grit pad, move the sander back and fourth several times until most of the shiny spots have vanished. Use a slow, steady stroke, applying slight pressure, and holding the pad always level with the board. Do not stop your movement at any time as this may cause a glass burn or sandthrough. An inexperienced sander might want to continuously start and stop the sander in bursts until you become comfortable with the sanders steady spinning motion.
Start down the middle of the board and work to the edge closest to you. Care should be taken to avoid catching your electric cord with the spinning disc. Also, be careful the board does not slip off its rack from too much downward pressure. This can be avoided by standing close to the board while sanding and also by moving the board back and forth a few times after placing it on the racks. This will allow the racks sandpaper to get a good grip.
After the deck has been initially sanded, turn the board over and repeat the process on the bottom. If sanding a board with a fin box, do the box area first. make smooth passes over the box while applying slight pressure. If the box is seated correctly, it will smooth over very easily with no burning or other problems.
When fin boxes are glassed, they sometimes float up in their hole, causing the outer box edge to be raised above the board's bottom. If the box was not seated properly, a very tricky problem may develop when sanding. Use a sureform to take the box down as close to the board as possible. Next, apply three or four firm passes over the box and on one side of the tail section only. More passes at one time may cause burning. Try to keep pressure on only the box and not the board.
Next, go to the other side of the board and repeat the operation for that half of the tail. If a light spot appears on the foam, the board has been sanded too far, and a new piece of patch cloth may be needed for repair. If yellowing occurs, the board has been burnt, and again repairs may be necessary. Slow careful work will avoid these problems.
If the board has a glassed on fin (or fins), sand the fin base as smooth as possible, but leave the fin itself until later. Inexperienced sanders should use a soft 5" pad with a drill for the fin base area. Make sure the buildup from extra rope along each side of the fin has been removed from the front and back of the fin. Sand these areas down to near the board's bottom level, but don't sand too fast or you will cause a burn.
Set the board on its rail with its deck facing you. Just touching the disc to the board, work the sander back and fourth along the rail with quick passes until the top edge of where the bottom and top glass coatings is reached (see Figure 02). Go around to the other side of the board (remember you always sand the board side closest to your body) and work the bottom edge of the rail to this same position. Don't worry about having the rail look smooth as this will come later. Flip the board over and repeat this same procedure on the other rail.