Pre drill your screw holes and squeeze in a poly urethane flexible adhesive,
like Sikaflex.
Coat your screw threads with more poly urethane.

...Ken

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.