Don’t use fiberglass resin and don’t confuse it w/ epoxy. Many here use the Old Timers formula, but I prefer the epoxy method.
Compared to the O-T method with multiple coats, the epoxy method is faster. I’d also say it’s more permanent, but have no empirical or comparison evidence to back that. However once your drill through such O-T treated wood to add any through-bolt fasteners, you just compromised the core as water can intrude via the bolt holes.
However, one thing you can do with epoxy that you cannot do with the Old Timers finish is you can drill a hole and fill it full of epoxy you so you have a permanent ‘plug’, once cured, drill thru it w/ a smaller drill for the bolt and your wood is still protected for a LIFETIME.
Examples of a transom re-do using epoxy & showing the bolt holes & ‘plug’ technique are here: https://forum.tinboats.net/viewtopic.php?f=3&t=47405