Welcome to the forum, and great that you are getting into this awesome sport!!!
Agree, if it's polyethylene go for silicone and stay away from epoxies and polyurethane resins.
Another option is use a hot-melt gun with polyethylene sticks (which are the most common ones off-the-shelf at Bunnings). This option should me more durable (pretty much permanent if properly done), however i't s also more laborious.
On another note, try to get a dagger board before you go on the water, even if it's DYI, otherwise the board will constantly drift. This applies only for the old-long boards like this one you have acquired.
Good luck!