How to: Using epoxy to secure a fixing.
The idea of this is instead of simply screwing the fitting on with a wood screw directly into the wood, the fastening is replaced with a self tapper that is screwed into a plug of epoxy. No part of the screw is in direct contact with the timber and the diameter of screw is in turn transformed to the diameter of the plug providing a far larger bearing surface. To use this method, the fitting is offered up to the spar and the fastening positions are carefully marked before removing. These are then bored out with a suitable spade cutter to a depth determined by the fastening. With a normal No 10 screw we would be looking at using a spade cutter of 15mm, but there is no hard and fast rule and is more dependent on the fitting, screw spacing, and number of fastenings. These holes are then filled carefully with Epoxy thickened with microfibers and flushed off. It is important to remove all air bubbles, and a prodding with a matchstick normally does the job. Once hardened the fitting can be offered up, suitable pilot holes drilled, and then stainless self tappers used to secure the fitting firmly.
Once again, the fitting is offered up to the spar and the positions marked. These are then carefully bored out to 75% of the depth of the fastening being used - a M8 x 50mm fastening will be bored out to a depth of 38mm with a 15mm drill bit. It is then necessary to drill out to the rest of the depth with a suitable drill size that will let the machine screw tap into the timber and pull itself in. Normally a M8 fastening will use a 6.5mm drill bit, but try it first in a scrap piece of timber.
The holes are all filled with an epoxy mixed with microfibers to a consistency that won't run out as you move onto the next hole. Again air bubbles need to be removed by stippling. Before the epoxy cures, offer the fitting to the spar and insert each of the machine screws through the epoxy and tap them carefully into the timber and tighten down. Be cautious about over tightening them as they will lose their grip on the timber. Clean off all excess epoxy and leave to harden.
It should be noted that it is not necessarily the case to use either of the methods on a properly designed spar fitting, but when a reefing winch or turn block is to be fitted to the side of a hollow spruce boom, it lends itself perfectly.