I am replacing thru-hulls on a 35 y/o boat. The ones currently there are, probably, not original but are getting long in the tooth (larger one's have "rounded" edges, which I am guessing is a result of corrosion eating at them). That said, thru-hulls appeared to be solid and at least one took hours to saw through (by hand, I don't want to gouge the hull with power tools). Thanks to whoever installed them for not using 5200, btw
Anyway, the previous thru-hulls had no backing plates. The hull fiberglass thickness is somwhere between 5/8" and 3/4", so there is a lot of glass. Hull is relatively flat where thru-hulls are installed. I am struggling with whether backing plates will do more harm than good. I have some teak for the purpose, which I can probably seal with epoxy. Still, over long term I wonder if teak would not deteriorate faster and then cause seal issues?