Wednesday, June 26, 2002 - Gluing the Bulkhead Scarf: I'm now ready to glue the two pieces of plywood together by applying glue to the bevels, putting the the bevels together, and clamping them. There are several ways to glue and clamp a scarf together. One of the time tested ways is to place the plywood on a level floor, tack the bevels together, and to pile weights on top of the scarf. This technique is covered in Sam Devlin's Book, Devlin's Boat Building, and in most of the other books mentioned in my section on "Past Boats." The technique I use is a little bit different. I attach an 8 foot piece of 2X10 to a pair of saw horses. I place the scarf along this piece of 8 foot board and support the other edges of the plywood with horses. Next I place wax paper under the scarf area so I won't inadvertently glue the plywood to the 2X10. I butter up the scarfs with glue, place them face to face, tack them, and cover them with another sheet of wax paper. Finally, I place 12 inch wide pegboard strips on top of the scarf and screw the pegboard to plywood and 2X10. This exerts an extreme amount of pressure on the scarf and makes a very strong bond. (At first I worried that the scarf would be weakened by the screw holes. I ran several tests by placing weight on top of narrow strips of the scarfed area and there was no cracking or breaking. I figure this is due to the inherent nature of the alternating plys that comprise a piece of plywood.)
|Items needed for gluing the scarf: 2-part epoxy glue, plastic cup, plastic spreader, rubber gloves, drill with screw bit, drywall screws, #10 washers, stirring sticks, wax paper.||Start by making sure the center of the scarf is placed over the center of the 2X10 board. Next place a piece of wax paper between the plywood and the board.|
|Mix up enough epoxy glue to liberally coat the scarfed area of the board.||Apply the glue to the bevels on both pieces of plywood using a plastic spreader.|
|Join the two pieces of plywood by placing the beveled edges together, lining up the edge of the top piece of plywood with the pencil line on the bottom piece. Tack one end of the scarf to the 2X10 board with a finishing nail.||Before tacking the other edge of the scarf to the support board I use a piece of angled aluminum to make sure that the scarf is straight. Tack the other side down.|
|Put a piece of wax paper lengthwise down the middle of the scarf. This keeps the glue from sticking to the pegboard, which will go on next.||I use a piece of pegboard about a foot wide to clamp the two pieces of plywood together. I drive drywall screws with #10 washers through the pegboard, plywood and support board. This gives me a very tight scarf.|
The finished product, ready to cure for at least 24 hours.
Notes: be VERY careful with the epoxy glue. Wear gloves and don't get ANY on yourself. This is very toxic stuff and prolonged or repeated exposure can cause an ugly rash and sensitize you for life. Wear safety or eye glasses as minimum eye protection. If (when) you get a small amount on your hands or face, wipe it off immediately using a paper towel soaked with white vinegar. (You may also clean your tools with vinegar.) Epoxy fumes are also not entirely wholesome and can cause respiratory distress if your work area isn't properly ventilated. (You can't see it in my shop photos, but I have installed a large exhaust fan in my shop roof which does a good job of sucking the fumes out of my work area.)
I usually work with 1/4 inch plywood and I haven't had any problems using this scarfing technique. This time, using 3/4 inch plywood, I broke off several screw heads while driving the screws. I assume the reason for this is that there is more resistance to the screw in the thicker plywood. I solved the problem by taking care to slow down my drill speed and stop when the #10 washer started to bend. More to follow.