Surfing Waves

I've got a nasty ding on my rail about 6 inches from my nose... I've got a Resin/Ding repair kit which instructs me to sand down the area first.

Any idea as to what grade of sandpaper I should get and how to know when I've sanded the area enough (and not too much)?

Thanks!

I used a very fine steel wool... (although this was never suggested to me by and surfer). I just polished it slightly and made it a bit rougher.