Working with resins and fiberglass is not all that difficult, a little messy at times, but not particularly challenging. And if you screw up, get out the grinder and do it again.
The CB on my Scot had been severely damaged by some prior owner. And then poorly repaired. It appears that the CB had come in contact with a rock on multiple occasions. The repair consisted of a ¼" bronze or brass rod bent to the shape of the rudder and then filled with thickened resin. Eventually, it all chipped away. This first picture shows the damage after some time spent gloriously sanding away bottom paint and whatever to assess the damage. You can also see that the glass was not well saturated with resin as it white.
Once all the failed repair was removed, it was time to start building up the end of the CB. Simply adding thickened epoxy would be a bad choice as another chance meeting with a rock would likely damage it. In this photo you can see the damage on the leading edge after the old repair was removed. The board below it is covered in wax paper and shimmed up to provide support for the new glass being laid up. The line on the board shows where the edge should be.
The second photo shows the edge after several layers of glass and mat were in place. There are 2 schools of thought here. One says start with small pieces and build up to larger and larger overlapping pieces. Another school says start with big pieces and then smaller and smaller. I went with the first option. The mistake I made here was using mat with epoxy resin. The glue that holds the mat together is styrene based and does not absorb epoxy resin all that well. Polyester resins work well with mat. For this application, it probably doesn't matter a lot, but I would do it differently in the future.
The lower half of the CB was then covered with a layer of 2 of glass. To make life easier I just let the glass hang off the end and trimmed it when it was partially cured. It was a bit faster to do it this way than trying to fiddle with the cloth to get it perfect or to wait until it was fully cured and spend more quality time with the sander. Once the first side was glassed, I flipped the board over and built up the lower edge with epoxy thickened with a high density filler and then encapsulated it in more fiberglass cloth. From that point on, it was a matter of adding fairing compound, sanding, more fairing compound and sanding until I got the shape I wanted.
Once it was faired, I painted it with Interlux 2000 to give it a nice hard epoxy surface and then I got distracted by life and a new boat, and it is still in the basement waiting for final touches.
For your rudder, cut the damaged area out and then feather back the fiberglass to a 12:1 ratio on both sides. Cut a block of hardwood like white oak to fit in the area that you cut out. Take a piece of wood cover in plastic wrap, wax paper, or packing tape and tape it to the side of the rudder. This will provide support for the wood block. Flip the rudder over. Glue the wood filler block in place with thickened epoxy. and then glass it in. When it is cured, flip the rudder over and do the other side. I would think about using just 1 layer of glass on each side at first and then set the rudder on edge and build up the edge by wrapping glass from one side to the other and build up the area. Then sand, fair, sand, fair, etc. On this repair I would recommend starting with a small patch first, just large enough to hold the block in place when it is flipped over and then begin with a large patch and work towards the smallest to fill in the area.
None of this is particularly difficult and a whole lot cheaper than buying a new rudder. The one other concern is the wet core. Go to the West System site and read the section on repairing wet deck cores, basically it is the same process for a wet rudder core. I did that on a rudder before I had a digital camera, so there is no record.
If you have read this far, you can get a discount on Total Boat products by using the discount code found in the Descriptions of recent Boatworks Today videos. You get a discount and Andy gets a small percentage of what you buy. Everyone wins.