Inject Non-Thickened Epoxy First
I know I said "thickened epoxy", and we'll get there, but first it is a good idea to inject clear un-thickened epoxy into each hole. By doing this you allow the less viscous, resin only, without thickener, to penetrate the surface pores of the core. this creates a better and stronger final bond.
In order to do this I inject straight epoxy into each hole with a syringe. I then let it sit in each hole for about 3-4 minutes and suck it back out with the syringe and squirt it back into the container where I add Cabosil to thicken it.
Mix In The Cabosil
Adding Cabosil, or the thickener of your choice, is easy but you can not make it too thick. It needs to drool off the mixing stick in a steady stream. If it goes drip, drip, drip off the mixing stick it's probably too thick.
Remember this slightly thickened resin has to fill a hole and let air bubbles escape leaving behind no voids. If you make it too thick you will have bubbles and voids. Not too thin and not to thick..
Remove Trapped Air Bubbles
Here I have re-injected the holes with my syringe. I did cut about .5 cm off the tip of the syringe which makes the hole bigger to accommodate the more viscous thickened epoxy.
The green stick you see is simply a piece of weed whacker line. This is a very simple and cheap tool for popping any trapped air bubbles and moving them to the surface so you don't get any voids. All you do is move it in, out and around the hole until you get no more air bubbles then slowly pull it out.
Tape Off The Deck
In this photo I have actually taped off the deck side to show what it will look like. You can use an X-Acto knife or a countersink spun between your fingers to get a clean hole. Taping prevents you from needlessly having to clean up epoxy residue which can be very difficult if not impossible once cured.
You will also notice the syringe filled with the Cabosil thickened epoxy and ready to go.
Wipe It Down
Of course if I were doing this on my boat I would have first pre-taped over the holes, like the photo above, & then used my countersink bit between my fingers to remove the tape from the holes. This method protects the deck and the countersink carves out a nice clean hole in the tape.
In this photo I simply used a rag with Acetone to wipe away any over flow. Use tape and it will go much smoother and cleaner. I simply forgot when photographing this..
The Tape Side of the Deck
In this photo the epoxy has cured and I have removed the Duck brand UV resistant duct tape. It seals perfectly and is not affected by the curing resin.
TIP: Before applying the tape, wipe the surface clean with Acetone or a strong solvent that will remove any grease or wax. The tape will seal and stick better. Gorilla brand duct tape is about the most adhesive you will find and also works well for this type of job.
#654 Bit - Cut-A-Way of Potted Deck Hole
After I removed the tape I re-drilled each of the holes with my 1/4" drill and then counter sunk the two holes that had been potted.
This is a picture of the hole I made with the 1/4" #654 Dremel bit. The hole is clean and precise with plenty of separation between the epoxy and the core. Both the top and bottom holes through the deck skins remained at 1/4" for a factory original look and feel yet this is much better than how it would have come from the factory.
#115 Bit - Cut-A-Way of Potted Deck Hole
This is the cut-a-way of the hole I cored out with the #115 bit. If you'll notice the epoxy potting is a little thicker than with the #654 bit. I don't feel this serves much of a purpose, & both will seal the deck from future moisture, but does require slightly more work in the over sizing of the top hole to 5/16" or what ever size you choose.
When you re-drill your holes, after filling them with epoxy, try and do a better job than I did of centering your bit over the original hole or your deck hardware might not line up again.
This re-drilled hole is off set to the left a little bit. Please be careful when drilling over sized holes as it makes re-drilling that much more difficult. When in doubt mark the center points of all your holes using the deck hardware as a template.
Explanation of Countersinking
So why on earth do I countersink or chamfer the deck side of the skins?
#1 it does a lot to prevent gelcoat crazing! By feathering the edge of the gelcoat, through the use of countersink bit, you are less likely to start a crack or craze mark in the gelcoat.
#2 By creating a bevel or countersunk recess in the surface of the deck the marine sealant or butyl tape has a cavity to fill. Without a bevel the hardware would compress the sealant to about 1/64th of an inch thick after the fasteners are tightened. Marine sealants do have some flexibility but not as much as you would guess. Follow me for a moment; Let's say you have a marine sealant that has a rating of 400% elongation before break on a 1/64" thick joint. Simple math shows you that 400% of 1/64" is only 1/16" of total allowable joint movement before a joint failure or leak starts. In the case of a stanchion base 1/16" is not much allowable movement before failure.
By countersinking around the bolt holes you increase the maximum thickness of the sealant, at the bevel, to roughly a 3/32" depth at it's deepest point. Using the same math as above 3/32" X 400% gives you a total joint movement before failure of 3/8". If a mechanically fastened joint is moving 3/8" you have more problems than just a deck leak! So 1/16" allowable movement before failure at 400% without countersinking or 3/8" allowable movement at 400% before failure with countersinking. Even a small bevel will drastically increase max allowable movement before failure. Countersinking to a mere 1/16" depth will give you 1/4" total movement around the bolt before failure. You don't need a deep bevel to make a large difference between a failure and a seal.
#3 Countersinking is really a no-brainer, and has many many benefits. For instance, you can install the fitting and tighten it down NOW with no waiting. This means you can seal deck hardware alone.
I am going to inject some strong opinion here so please move on to the next photo if you don't want to hear it. I truly dislike and disagree with the Don Casey "two step", "wait to tighten and form a gasket" method of bedding deck hardware with a "curing" marine sealant. In my opinion this method is probably one of the leading causes of deck core rot on the planet. Believe it or not but I'm not a conspiracy theorist! I do however feel as if Don Casey wrote that technique to guarantee boatyards future revenue. This is NOT to say that when done "properly" it won't work just that it is VERY difficult to actually do this properly and many a DIY and pro have messed this up and ruined decks as a result.
Think about it? If the sealant cures, to form a "gasket", and you then move the bolt while tightening it down on the "second step".... you will likely lose. If you create a gasket too thick, it cures, and then can't get the mechanically fastened hardware tight enough against the deck because of compression resistance, the hardware will move, and also fail, so again.... you lose.
Sorry, I mean no disrespect to Mr. Casey, and he writes some great stuff, but this method is perhaps some of the worst advice I've ever read in any sailing/boating related book. I have witnessed far too many cases of core rot due to DIY & pro "two step" bedding procedures than I would have like to.
So where did I lean this countersinking method??? Hinckley Yachts. Sure this extra step takes all of about 30 seconds per stanchion but apparently it's too much "extra" work for most production builders.
OK I'm done with my rant...
In this photo I used butyl tape as a bedding compound, it's what I use on 90% of my deck fittings. I loosened the nut and pushed the bolt up to show how much allowable movement can be had with a countersunk hole.
If you look close you'll see that the outer edges, where the bolt head meets flat deck, the sealant has begun to fail or failed entirely yet the butyl in the countersunk recess is still stretching and still there! The butyl is also still well adhered to the threads from being compressed by the bevel when tightening the bolt.
Not Much Stretch
This bolt hole has no bevel at the deck level. It was loosened and raised the same amount as the previous bolt, to illustrates how much difference in allowable movement one can attain through the simple step of countersinking.
When I tightened this bolt down most all of the sealant squeezed out leaving very little of it to stretch.
Here you can see how the countersunk hole on the right has forced & compressed the butyl against the bolt and into the threads creating a great seal. The bolt on the left was not countersunk and has very little sealing surface area around the bolts shank or threads.