Countersink The Flange Side
In this photo I still need to countersink the flange side of the bolt holes slightly. The reason you want to do this is to create an o-ring like effect of marine bedding compound surrounding the threads. If you don't countersink most of the sealant will squeeze out when you tighten everything down leaving very little to prevent a leak. The premise and design is that water should never even get to the top side of the backing plate but just in case it's best to countersink slightly and make and create an o-ring effect.
Countersinking any hole, where marine bedding compound is to be used, is a good idea especially with deck hardware. This slight countersinking makes a nice water tight gasket and also prevents crazing of the gelcoat from the sharp and abrupt edges a straight drilled hole creates!
Flanged Adapter With Bronze Ball Valve Ready To Install
At this point I have applied some Sikaflex 291 marine sealant and am ready to install the flange. If you look very closely you'll see that the flange has been slightly countersunk so it's ready. First apply the sealant to the flange, in the fashion shown, then generously circle the machine screw heads with sealant and feed the bolts up through and into the boat from the outside.
Now carefully set the flange onto the bolts. Thread the nuts and washers onto the bolts, finger tight, and then climb back outside the boat. Slather the thru-hull/mushroom-head's threads with sealant and apply a generous amount to the head or mating surface & thread it into the flange and climb back into the boat.
To tighten the nuts, with only one person, I find it much easier to use an impact driver. I use an 18V rechargeable Ryobi that was cheap and works well for the purpose. Be careful with the impact driver as you don't want to over torque them!
The impact driver is a great tool that will tighten the nuts without having to have a second person outside the boat with a screw driver. Once the nuts are tight grab the step wrench, and a large adjustable wrench, and tighten the thru-hull into the flange from outside the boat.
Once everything is tight clean up the ooze from the sealant, including any that oozed out up inside the through hull and around the exterior. Be sure to clean excess sealant from the counter sunk bolt heads so you're ready to fill and fair the exterior with a vinyl ester filler such as 3M Marine Premium Filler.
Marine Premium Filler is a unique vinyl ester formulation designed for marine filling and fairing applications above or below the waterline. I would avoid the use products or fillers like Bondo or Evercoat below the waterline as they can absorb moisture. Make sure you're using either an epoxy based fairing compound or a vinyl ester fairing compound to minimize moisture intrusion.
So why do I use Sikaflex sealants or bedding compounds rather than 3M products? That's an easy one for me to answer. In my experience I've found it less aggressive in it's adhering properties than 5200, something I really like for future repairs. I've had vast amounts of experience, over 30+ years, with many marine sealants and have really grown to like the Sikaflex products.
If there is one thing I've learned, in 30+ years of boating, it's that NOTHING on a boat is permanent, not even a keel joint, so there is absolutely NO need for a sealant as permanent as 5200. In my opinion 3M 5200 was invented to ensure future boatyard revenue but remember what my opinion is worth.
I've seen 5200 destroy gelcoat, port lights, hatches and teak among other things. There is only one product not allowed on my boat and it's 3M 5200. 3M 4200 was a move in the right direction, being less adhesive than 5200, but for many projects it's highly adhesive properties are still not necessary and it's still overly tenacious in it's grip.
Now don't get me wrong I'm not saying, in any way, that Sikaflex comes apart like butter, it does not, but it comes apart easier 5200. Try Sikaflex 291 & you'll like it's adhesion, flexibility and long life.
Exterior View of Installed Thru-Hull
As you can see in this photo the silicon bronze bolt heads are countersunk and most of the excess Skiaflex sealant has been wiped away.
The next step is to fair over the bolt heads with 3M Marine Premium Filler to get a nice smooth hull with no visible bolts.
After the fairing step I also did a complete barrier coat of the area for added moisture protection using Interlux Interprotect 2000 series barrier coat. This step is not 100% necessary but I wanted to protect and prevent any water intrusion into the recessed bolt holes as I could and also protect the hull where I had sanded thin the original gel coat..
A Few Words of Caution !!!!
*WARNING, WARNING, WARNING* if you are one of the "cheap skate types" who think "marine rated stuff" is a bogus marketing ploy DO NOT stop reading here.
This is a photo of a one year old YELLOW BRASS Home Depot ball valve installed by the previous owner. When I asked this seasoned boating veteran why he used a ball valve from Home Depot he said; "it was only in a pinch while coming up the ICW but after it was installed I forgot to replace it with bronze one".
This "Home Depot" valve was in use for about 8 months, of total in water time, and the ball, inside the valve, is completely GONE, NOT THERE, CORRODED AWAY! This valve was literally weeks away from catastrophic failure of the boat sinking type.
*PLEASE DO NOT USE ANYTHING BUT MARINE BRONZE OR MARELON ON YOUR BOAT'S SEACOCKS OR THROUGH HULLS! BE SURE WHAT YOU ARE BUYING IS MARKED UL MARINE.
This is What the Exterior Looked Like
This is after only eight months of use! Again DO NOT cheap out on ball valves!!!
The Finished Product
This is the finished product after the first seasons use. This seacock happens to be for a head intake..