Bob

re: Thinning the gelcoat

I understand (but have not yet tried this) that you can heat both the gel and the surface (25 degrees celsius) and that thins it out. Sounds worth a try (but I suggest you work fast and only test it with a small quantity first!!)

Trailing edge

This is alway tough. I usually open the nick out a bit (particularly if it's small) with a small metal file. Then I use packaging tape and/or some smooth plastic along one side (parallel to the trailing edge) to make a molding surface. Then I place the board so the tape is on the bottom and fill the nick and tape with gelcoat. If you do a reasonable job with the 'mold', you shouldn't need to sand too much. Sorry this is hard to explain in words. Maybe someone else has a better method/description too!
cheers