You'll need to get UV stabilised non-yellowing epoxy. I use this in all my repairs. Australian made and the 2:1 ratio makes it easier to get the mix spot on:
boatcraft.com.au/Shop/index.php?main_page=product_info&cPath=15&products_id=1They also do a Crystal clear resin you might want to look at.
Sand the bottom of the board smooth and put some resin on any bare patches. Give it a light spray with SuperCheap acrylic white afterwards if you really want. It doesn't need to be perfect and will get trashed by the board bag anyway- if you use one. The main thing is just to keep the water out.