I know other people do it, but screws in fiberglass is a really bad idea.
They will not take a lot of force and you expose the fiberglass to potential
damage. It is far better to bond in backing plates and bolt to them . This
spreads the fastener load across a larger area. and sealing the fastener
with deck sealant will prevent leaks.
Steve

wrote in message
...
I am about to finish up a moderate fiberglass deck repair and am
curious about fastening the mounts for the post seats. After I put
the carpeting back down, how do I put in the screws without breaking
the waterproof seal in the new fiberglass? Thanks for the help.