Bue, 36-60 Will be fine for your surface prep. Keep in mind that the Urac will have leave a colorful glue joint behind, SO, the ruffer the sanded joint,the more visible the joint will be. For this reason I would say 60 grit would be best,Thats what I use for all my glue joints. I also use epoxy as that is what I have and it works great. As Macbow said,and you already know,Urac is not for a wood to glass joint,wood to wood only.
Good luck
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