1. Go slow during the repair.
2. Experiment with the materials away from the board, do a little testing on a piece of wood.
3. When you are confident with your materials, then tackle the board job.
MARINETEX it's a two part filler that can be sanded, it dried even under water, and can be used to repair a cracked engine block!
For smaller repairs, you can try the little sticks of two part material they sell in the plumbing section at Home Depot. It's called Ding Stick in the surf shops. This stuff looks like a candy Bulls Eye.... dark circle filled with a white center. It's a little log of the stuff. You cut off what you need, smash the shite and grey together, and it hardens in about 10 minutes.
It's much easier to remove excess before it hardens with both of these materials. They are very hard when dry. You risk sanding a hole in the board, while trying to sand down the repair.
Appliance Enamel Spray Paint, available in the suburbs, works great for a finish coat.
Don't use BONDO, it's made for cars and needs to be painted over I think. It's also not very strong as I remember.
I hope this helps.