I've been sailing for a few years now, but haven't really done any "major" deck work that involved drillings holes through the deck. I was wondering what is involved in correctly drilling and sealing a screw hole through the deck. What is the bent nail trick? What type of epoxy is best to use?
1. Don't drill any holes that aren't ABSOLUTELY necessary!
2. Use polysulphide as the sealant
3. Use enough polysulphide so that when the screws are tightened some of the polysulphide is expressed.
4. Do not tighten the screws all the way. Let the polysulphide harden and a few days later fully tighten the screws. By doing this one does not express all the polysulphide and there will be a gasketlike seal.
I am sure that there are MANY solutions to this problem but here is the procedure which we have used on Botany Bay without problems.
1) Drill the correct sized holes for the piece of hardware and install a proper backing plate on the backside of the deck (no sealant at this point)
2) Overdrill the hole 1/4 - 3/8 inch larger than the fastener or 1/2" whichever is larger (I have trouble filling the hole if it is smaller than 1/2" with any repeatability
3) Come in from the backside and overdrill the hole by another 1/4" but only go through the back skin and core making sure that there is no core material left on the backside of the outer skin which could wick moisture into the core. (I find a hole saw works quite well for this application) If the core is moist already drill it out a little larger.
4) Get some really good duct tape and put it over the backside of the deck, make sure there is not an interior removeable liner or some such, you have to have access to the underside of the deck or the epoxy will go everywhere.
5) Fill the hole from the top with slow cure epoxy. I use West Systems Epoxy and go one hardener level slower than the min recommended for the temp. The problem is that there is a significant amount of heat build up and it can bubble which won't result in a good seal. I also add West Systems Filler #404 (High density/High Strength filler)as much as I can and still have it flow easily (This also helps to avoid the overheat problem and provides strenght for drilling).
6) Wait 24 hours, then drill hole correct sized for the fastener, bolt up the backing plate again to make sure everything fits.
7) Tape the deck with 2" wide tape to protect the deck from the sealant. A trick here is to put the tape down before installing the component, then trim around the component with a knife, then remove the component and take off the tape which is under the component.
8) Use a polysulfide type (Lifecaulk for example) sealant with lots of extra coming out around the edges to bed the part. I put the sealant on the part with the bolts sticking through to keep the mess inside the boat under control. Put some sealant down the bolts but not too far (about a deck thickness or something)
9) Snug the part down but not too tight, wait 3-5 days until the sealant cures then tighten the part down and trim the sealant. Remove the sealant and tape together.With the exception of chainplates the above has always worked well. In the case of chainplates I use 3M5200 slow cure and rebed every couple of years.
We don't have ANY leaks in the deck to my knowledge and we live on the boat. If we do find a leak we work on it rapidly. The previous owner did not and we ended up having to replace about 1/4 of the deck core over the last 10 years. It was definitely not a pretty job but the boat is now dry. There are several hundred holes like this through our deck and all of them have been done this way.