BAL primer on Marine plywood

Hi, I've put up a shelf behind my bath and tiling over it. The shelf is made from marine ply. I have been instructed to use BAL primer on it before applying my adhesive. I have 2 questions: i have seen online that i should not apply the primer to the area i am laying the tiles, but to all of the edges and the underside. is this true? when applying the primer, should i apply it straight or 1:1 diluted? many thanks, rob