Yes, you need a good seal with the upper skin. That's why I use a forstner bit to drill up from the bottom, as it makes a very clean, flat surface on the upper skin.

Then, the West 6-Ten epoxy has such a wonderful consistency, if you make sure to get some on the top of the tube you are inserting it will bond very well to the upper skin (as well as the rest of the hole that's been drilled.

:: Brendan,
::
:: Thanks for the great idea. I have sealed hardware the
:: traditional way over drilling and it seems to work but
:: then you have to layout your hardware again which may
:: cause you to drill into the core partially and you can
:: not find the original holes.
::
:: My question would be though, if you do not get a good
:: bond between the top of the tube and the top skin of the
:: boat, would the fastener not leak into the core? Any
:: tips on how to prevent this or check you have a good
:: bond?
::
:: Thanks,

:: Don
::
::
::
:: :: 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