No need to waterproof, just stand your board upright to dry after you've ridden it in the wet and dont use it as a surfboard.
you could cover the everything except the edges with newspaper and tape, then spray it with polyurethane?
If you're really worried about it, ring around and find a place (last place I saw it was at a 4 wheel drive store) that stocks a product called PlastiDip. What it is, is a kind of melted rubber that hardens. RC hobby enthusiasts use it to dip the water sensitive electrical equipment on their cars when they want to run them though puddles or in ice. You can get ones that come with a brush (so carefully apply it to the areas you want waterproofed), they also come in a variety of colours. Consult YouTube to see how it works and if its for you.
outdoor furniture oils from bunnings work best...oil penetrates, doesn't create a skin that will bust up and scratch off.
Or just save yourself to money and towel it off when it gets wet ....for timber to properly water log it would have to sit in a bathtub for days on end. plus that decks bamboo so you've got no worries
just dry it off by hand, or dont ride it in the rain..... no rain riding means not wet board
cheapest way would be to smear some butter or marg or what ever is in your old ladies fridge all over the edges of ur board.....ur mates will never believe u if u tell them.
lol ......... just rub some bees wax on the exposed edges, make sure the board is dry b4 u do it n sand it first, of cause the other way to do it is to LOOK after it...... dont leave it in the car boot for ever,
...dont leave it in the sun for 2weeks
... dont let ur mates borrow it