We had cracks and a hollow area on our C30 rudder. I could see water in the rudder looking down the rudder tube. Took the rudder off and home for one winter. Wrapped the forward, rear, and bottom edges with epoxy using 6" and 4" glass cloth band. I prepped each area and then added the cloth ( actually bought 6" wide ribbon and 4" wide ribbon so I wouldn't have to deal with unraveled edges). Then I wrapped the rudder with saran wrap taking special care to NOT have any fold overs or wrinkles. The saran allowed me to keep the resin where it belonged without drips and more importantly, kept the ribbon tight against the rudder. After curing, the saran comes off easily. My hollow spot was actually a manufacturing defect not freeze damage. Filled the hollow with fairing and then over layered with cloth. Faired all, sanded and painted with barrier coat. BTW, when i put the rudder back in, I checked the top surface of the rudder post bearing area by blocking the rudder post just flush with the tube bearing surface. Mine was way off. Sanded flat and parallel to the top rudder post surface, added a thrust washer to take up the space and finished the job.