OK, after 6 weeks of weekend trial and error (along with about $200 of error) here's the solution - figured somebody might be facing something similar and may benefit from my solution. Throughout all of this, I had the "solution" of cutting out the headliner which would have been acceptable with a teak
blank to cover the hole but I really didn't want to do that if I could avoid it just because that was something I was sure I would not do sufficiently well to make it look like something I could be proud of.
I first discovered the Toggler Snaptoggles
. They come in stainless steel
and are rated to hold up to 356 pounds in in 5/8" drywall. I was planning on using 4 of them in 1/2" fiberglass
which, while I don't know for sure, I figure is stronger than drywall so it should be good.
Since I can't get to the backside of the bolts I've been drilling out the existing stainless steel
screws on the stanchion bases - 4 per base. Good times.
I say "was planning" to use the snaptoggles because it turns out the depth
of 2 of the 4 holes was not sufficient to allow the snaptoggle to rotate behind the deck
and be used - it was only about 1/2" deep and the toggle itself is over a inch long so it would not even go all the way into the 2 most inboard holes (1/2" think fiberglass, 1/2" deep hole). Much swearing occurred and we decided to retreat, reorient and reattack. After a few beers of course.
The next thing we found was the PlusNut blind fasterner
. Amazingly, these also come in stainless steel (although they only had 41 in stock but I only needed 34 total). I tried these out last weekend between rain showers and they fit right into the most inboard holes perfectly and allow the 1/4" bolts to fit. Again, I am not sure how strong these are in 1/2" fiberglass but I'm going to bet the lifeline on it that it's strong enough. Especially since about half the bolts I found had no backing nut on them at all any more (if ever) so I'm 100% certain I am better off than I was before I started all this. Looking at the chart for these (in kN so I had to convert) it looks like I could expect at least a couple hundred pounds of force to be supported - and with 2 of them that should do the trick.
So now I am putting the snaptoggles in the outboard
most 2 holes as I believe these will actually give me stronger results (and I already have them anyway so I might as well use them) and then I'm using the PlusNuts on the inboard 2 holes.
All together I am pretty sure this solution will provide a much stronger installation
than I had, maybe even better than the day the boat rolled off the production line. And they won't leak.