Originally Posted by Cmtravels
What is the best way to join the two pieces together?
I'm a bit late to this party but there may be some tricks to pass on. Limiting the 'repair' you have to do is the real challenge on a job like this and most instructions assume you have a friendly angel who will hold the piece in place while the resin hardens! Here is my suggestion - the drawing's a bit technical, but it's meant to be a slice through the joint between the old and the new at any point.
1. Cut the filler piece to the shape of the hole (that makes the repair as small as possible), or vice versa if easier. Taper the inside of the cut edges as much as possible - a 1 to 10 slope would be perfect, but the 1 to 3 taper shown will do. Rub down the inner surface of the filler piece and original shell with something like 80 grit sandpaper for several inches back from the joint.
2. To hold the filler piece in place until laminated, drill holes through the middle of the joint and fit small bolts with penny washers to keep the two surfaces exactly flush with each other. Do this with care as every misalignment will show later. Two or three bolts along each side should do, but more may be needed. Before installing them, coat the bolts and washers in wax to make it easier to get them out if resin gets on them. You can apply packaging tape across the outside
of the joint between the bolts to stop any resin leaks
- this is helpful if you haven't cut the two pieces perfectly the same.
3. Lay thin strips of fibreglass mat or tape along the vee of the joint, between the bolts. Let these harden and they should be enough to hold the filler piece in place, allowing the bolts to be removed. Do not be tempted to leave the bolts or nuts in place, as that6 will cause much more work later.
4. Finish laying up fibreglass mat or tape to more than the thickness of the original shell and overlapping the repair on the two sides - onto the areas that you sandpapered before.
5. You are now left with a small 'repair' to do along the joint on the outside to make it cosmetically acceptable. A little grinding out and adding bondo may be needed to get a smooth surface to finish. Never be tempted to widen the area that needs cosmetic repair as that makes more work to do.
In case it's not obvious, this technique can only be done from the inside - well, it could be done from the outside but then the cosmetic repair afterwards would be a huge problem.