I just made backing plates out of cutting board
material nearly 1/2" thick for a new knot meter thru hull on my boat. I used 3M 4200 to bed the plate to the hull but was tempted to use 5200 or epoxy or even Marine-Tex but relented to 'advice'. So far so good and my boat is still floating without any leaks from my work (yet). I would bed the bolts with a thickened epoxy and maybe bed the whole mess outside the hull with 4200 to protect the epoxy from sunlight. The general consensus is that putting self tapping screws directly into fiberglass is a bad idea as it encourages water to infiltrate the fiberglass (assuming your boat is) resulting eventually in loss of rigidity. That is why Ross suggested you "Run, don't walk..." to a new rigger. A rigger may know wire gauges bit is not necessarily a fiberglass expert. Nor am I! I also replaced the aft chain plate assembly on my boat several years back and on the Tartan 27 (1967, so a little older than your boat) the chain plates are through bolted to a hunk of wood (I used the S. American wood: Ipe) which is totally glassed into a knee wall. The old original plywood that had been glassed in had rotted to soil after 37 years of soaking up moisture. It is important to re-bed your chain plates if you can. Several years on that repair with my only regret being that anything SS that is totally glassed in (eg., no air) are prone to fail much faster than if the bolts are visible. I suppose this will eventually be someone else's problem.
