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fix screws into the GRP

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If you are using self tappers drill a clearance hole though the gel coat to minimise chipping and a generous sized pilot hole for the screw. That means if you are using 4mm (no8 gauge) screws and a typical set of drill bits a 4.5mm hole through the gel coat and a 3.2mm pilot hole or even 3.5mm if the grp is thick. I also like to put the screw in with a little epoxy adhesive (but not if some lubricant has been used)
If you have access to a more comprehensive set of drill bits then maybe don't make the clearance hole quite as much as 4.5mm and make the pilot hole 3.3 or 3.4mm.

I think if I was using larger screws than 4mm/no8 I would consider tapping a hole for a machine screw say M5 or M6. I'd still put them in with a dab of epoxy though.

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