I can't speak too much of ways to repair the bottom of the board, however when I cracked my topsheet, I used a product called SunCure to fill it in. Worked super well. Just a thought.
I have used marglass which is a fiberglass bondo. It seems to work pretty well and can be easily sanded smooth again. You can get it at autopart stores.
I just use bondo if you have to fill anything. On a few boards I have, I have completely rebuilt the fins with bondo after they were shredded off. If it is a crack then you should use some sort of 2 part epoxy or like sbt3 said with the marine fiberglass.
I cracked the edge of my board about a month ago and repaired it with a 2 part epoxy. Ive ridden it a bunch of times since and have had no problems. I got it from west marine...heres a link.