I use marine-tex on my old dings and/or cracks. Sand the area down so it will stick. Put thin duck tape in the areas where you don't want that stuff clinging to the board, rinse or clean with acetone, and putty away.
Be sure to take the tape off before it dries.
Then use razors to shave the clumps down, sand it to make it smooth. Spray paint over if the color is ugly.