I'd recommend using west system epoxy materials to make repairs and get to a smooth surface - then you can either paint or go back with gel coat (make sure you understand if hardens in air - and scrub the repaired surface with soapy water first to remove the amine blush from the epoxy).

Use colloidal silica with high density adhesive filler for big dings or ground off bottoms, then colloidal silica and fairing filler or low density filler for small dings and fairing out.

Small chips in the trailing edges of boards can be easily repaired by placing a piece of masking tape on one side, blobbing on some gel coat and then putting a piece of tape on the other side to flatten it out a bit and prevent runs. A little work with sand paper the next day and you have a nice finished repair.

Chris.