I have a 4x6 fiberglass blind. The windows slide up rails on either side and stay up with a Velcro strip at the top of the window attached to Velcro on the blind wall above the window. A couple of the fasteners on the window rails have come loose and I want to either replace them or fix them. Before I rip the current fasteners out of the fiberglass, can I use some sort of epoxy to reset them or do I need to replace them? If I need to replace them, do I need a bigger size than the current size since they likely chipped away the fiberglass inside the wall?

I would glue them back with Gorilla Glue and keep rocking.

There is probably a wood support strengthener between the inside and outside fiberglass where the window is fashioned. Does the fastener have an inside and outside, as in a small bolt all the way through? A pic would really help.

it really depends on the fastener used. Oics would help a lot. Plus knowing the structure around the fasteners.

Unfortunately, I'm at home now without pictures. But no, the fastener bolt does not go all the way through to the outside of the blind.

If it's like mine, and it sounds like it, my rails are riveted and then fiberglassed over on the outside. Havent had any come loose yet though. I'd drill out and re rivet it and seal back up with good silicone,

Sounds like my blind as well. I just did a refurbish, mine has a metal frame sandwiched between the fiberglass. It was easy to drill out the old rivets and add new ones. If the old hole is too big just seal it up and use a metal bit to drill a new hole.