Macroscien said..
Can we use just ordinary sugar instead of custard sugar?
Maybe remain cavities in resin remains even bigger for better grip?
Castor sugar. Custard sugar would be all eggy and stuff

Try it on something first. I tried different grades of grip, and I think castor sugar was about right. The larger the grains the better the grip, but you are going to remove some skin if you come off and slide across the deck.
Your choice.
* no I didn't edit my post after I made a spelling mistake!