The following article represents one person's opinion. The webmaster is an experienced restorer and I disagree with the use of epoxy glue for making scarf joints. I use either resorcinol or Weldwood Plastic Resin glue, depending, generally, on whether the joint is above or below the waterline. The following articles in Wooden Boat magazine present other points of view on this subject: issue 86, page 109; issue 119, page 98, issue 136, page 94
Scarf joints are often used in boat building. A scarf joint is a long angled joint. The long angle provides a lot of "glue surface" for the joint, making it very strong.
A scarf joint is made by joining two pieces of wood having tapered, beveled, or chamfered ends which over-lap together, as opposed to a butt joint where squared ends of the mating pieces simply butt together. Scarf joints are used to make longer members where single members of sufficient length are not available or are too costly. Both solid wood pieces and sheets of plywood can be scarf joined using epoxy. With the proper cutting and gluing methods, such joints will be amazingly strong, exceeding that of the joining wood members.
With scarf joints, it is preferable for the two pieces to fit as closely as possible for ultimate strength. However, minor irregularities of fit are acceptable since the epoxy will have the ability to bridge these without significant loss of strength.
The amount of taper with a scarf joint is usually stated as the ratio of the thickness of the joining pieces to taper. Common ratios are 1:8 ~ 1:12; that is, the length of the taper is 12 times the thickness of the members. In other words, if the wood were 1" thick then the scarf would be 8" long.
In practice, the higher ratios are stronger, while lower ratios are weaker. However, experience shows that in thicker members (especially those not subjected to great bending stress), a ratio of 1:8 is very acceptable.
There are a number of ways to cut the scarf joint. You could use a tapering jig on a table or band saw. Adjust the jig for a 1:8 ~ 1:12 cut and taper the ends of each piece of stock.
A simple jig can be built to use a hand plane or router to cut the angle. The jig is built with angled sides that produce the scarf ratio. The jig and stock is clamped to the bench, and a plane used to trim down the stock. The plane should be held at an angle to the jig, so that the sole in front of the cutter rides along one side and the sole behind the cutter on the other side.
Gluing the scarf joint is not hard, although some people have trouble with the joint slipping. To prevent that, mate the two pieces of wood together without glue and clamp them temporarily. Now drill a 1/16" hole from the back in the middle of the joint. You only have to go deep enough to start the hole in the second piece.
The gluing principal is to provide as much glue, as the joint will absorb in-order to prevent glue-starved joints. Coat the two joints with straight epoxy and let set for 5 or 10 minutes. Then recheck and apply more epoxy until dry spots no longer appear. Before the epoxy sets up, apply another coat of epoxy mixed with filler to a syrupy consistency. Apply the mix to both mating surfaces and allow it to set for 10 to 15 minutes.
Mate the two pieces of wood together and drive a finishing nail into the drilled hole. Now apply the clamps and pressure. When using c-clamps, use small blocks of wood to help spread out the pressure of the clamps, and use at least two clamps, one on either side of the "center nail". Do not apply too much clamping pressure; you do not want to force all the epoxy out of the joint.
Scarf joints that will be put under stress when unclamped should be allowed to cure for 5 to 7 days before removing clamps. This will allow the epoxy reaches its full strength. In circumstances when temperatures drop below 60oF at night, or for other long periods, the clamping time should be increased as at lower temperatures curing slows considerably.