A common practice for protecting deck core from rotting at fasteners is to "epoxy" the holes. Usually, this involves: a) drilling the hole, b) augering out balsa with a bent nail or something similar chucked into a drill, c) filling the whole mess with epoxy, and then d) redrilling the fastener hole through the epoxy plug.

I haven't liked this method--augering balsa is not always easy and it's very hard to ensure a full seal against the elements. I've found air-gaps and exposed balsa even after doing the traditional approach carefully.

I've been using a method that is easier and more robust.

I use G-10 tube (sourced from McMaster-Carr) to seal the holes. These tubes come in all sizes and you can easily find tube with 3/16", 1/4" and 5/16" ID. 1/16" wall thicknesses makes the OD easy to find drill bits for (see below) and is a good, robust tube.

To seal holes through cored deck I:

1) drill the appropriate hole(s) for the fastener(s). 2) from below, using a forstner bit (which makes a flat-bottomed hole), drill a larger hole for your G-10 tube up from the bottom just until you get to the upper skin of the deck. Try to leave a nice clean upper surface. 3) Insert G-10 tube and mark with pencil. Extract and cut to length with dremel or hacksaw. 4) Bond into place. I like the West "Six 10" epoxy which comes in a tube with both resin and catalyst and mixes immediately to a very satisfying and drip-free consistency.

Once the epoxy cures, you have a hole that is fully sealed from the elements, and also very well protected from compression.

A couple notes:

- Forstner bits work best, and can be hard to find in other than 1/16" increments.  Thus, consider the OD of your tube when ordering.
- Deck thickness can vary a lot in our boats.  Beware of measuring one hole depth and then cutting a bunch of G-10 pieces--you might make some that are too short.
- If I have lots of holes to do, I cut plenty of G-10 pieces that are too long, and then carefully shave them down later (Fein multi-master is the tool for this).
- Gently countersinking the holes from above will make a little area for the polysulfide to live and make a better seal for your fastener.

I've found that when fasteners bedded in this manner leak, you know right away from rusty washers below. This is because all the water comes right down the tube instead of slowly getting absorbed into the deck. This makes it easy to stay on top of leaking fasteners, and if you don't mind the rust stains, you can ignore them without damage to the deck.

And as always, don't use silicone or 5200 to bed your hardware. Silicone will not seal effectively and is a contaminant, and 5200 is first and foremost an adhesive. LifeCaulk or 4200 are the best commonly available caulks in a tube. Butyl tape appears very promising as well.

--brendan