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Grady, if you are going through 1/4 inch if fiberglass into 1/2 inch plywood, Any screw that goes in more than necessary to penetrate the plywood is just loosening the grip the screw has on the plywood. Think of it this way, if you used a 3 foot screw, by the time you got the screw seated the hole would be worn out. Same thing with a 4 or 6 inch screw just not as severe. I would use a screw 1 inch plus the thickness of whatever you are screwing down and use plenty of 3M 5200 on the screw holes and the bottom of the bracket. After that 5200 cures it won't go anywhere.