Finally, take your sandpaper
and sand down the resin/fibreglass. Start with a rough grade
and gradually go lighter and lighter until it's baby-smooth.
Be very careful whilst doing this (there's no rush), as
you don't want to sand down the undamaged parts. If necessary,
repeat the whole procedure a few times until the ding is
filled in and flush with the rest of the board.
*Health Warning* - when sanding down resin, always wear a protective
face mask. It is also a good idea to do the sanding outside, with
a bit of wind to blow away the resin dust, as it's nasty stuff.