I always suggest to get the surface as clean as possible and to oversize the Gator Grip such that it's not on the bumpy surface of the swimdeck. If water can get up underneath, the glue will start to breakdown and allow the material to lift up.
As for cleaning get off as much of the old material and glue as possible by hand and once you have the old material and the thin film backing from the sticky back, then use Acetone/Goof off to get the remaining glue off. If you get too anxious with trying to get off the material, you'll just have a mess.
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