Over-Drilling holes

[MacGyver]

I just wanted to take a moment to talk about fastening methods.

In the industry, we over drill "Cored substrates" meaning drill your first hole, the proper size, then drill it with a bigger bit, fill with epoxy to create a impermeable hole, then redrill original size, then use caulk or sealant of choice, then fasten item down.

Why don't we normally over drill non cored substrates?
Well, A core is something like Balsa, or foam, or plywood, etc. These things tend to soak up water, and the point is to keep that from happening.

A normal fiber glassed area, with no core (not every section of a boat is cored) will not absorb enough of the water because it is not wet all the time. It will typically spend more time dry than it will wet.

A bottom of a boat is not on the same principles as the deck or topsides. Some have a core, some don't. When drilling the bottom or submersed part of the boat, for instance a Thru hull fitting, you should drill to proper size, then seal the edges in EPOXY.
I recommend a slight overdrill, 1/16 of a inch, so when you put in the fitting with caulks, etc, it will have some between it after the epoxy has set up.

The whole point though overall is to seal up the edges that is within that hole, so when it leaks, which it will, it wont be absorbed into the core material.

I hope this helps people out.
Ask any questions and I will do my best to answer them.

Mac

[Salty19]

We have to remember there is a micro-balloon slurry core under the deck.  It does not soak up water like balsa, but if water gets in there, and freezes/expands, damage will occur.   It will delaminate the core and glass, weakening the structure.

I don't recall if the hull is cored with this slurry or not.

Safe bets is to always seal holes with epoxy to avoid hidden problems down the road.