You can read the
@Maine Sail article on bedding deck hardware but it's a little challenging getting through the rants and discussions of how others do it wrong. It's unfortunate there is so much clutter because the description is really well thought out with some very helpful illustrations. I know I'll probably get the wrath of the faithful upon me for saying anything bad about marinehowto.com but it gets a bit much sometimes. (sorry Rod, just my 2¢)
Here are the main steps but read the page to get the details.
https://marinehowto.com/bed-it-tape/
Here’s how I bed deck hardware with Bed-It Tape:
Step 1 – Countersink the deck holes
slightly and then clean everything with Acetone or a similar fast evaporating solvent. Be sure your cleaning solvent is compatible with your decks finish.
lots of other confusing stuff in the middle ...
Step 2 – Wrap Bolt Heads – You’ll want to rip off a small piece of butyl and knead it into a string then wrap the underside of the bolt or machine screw head.
Step 3 – Press the through bolts into deck hardware with some good pressure.
First though make sure the hardware is clean, Acetone works well, but if Silicone was previously used you’ll really want abrasives as Acetone won’t touch dry silicone contamination.
Step 4 – Strip some more butyl tape and make another round string. Wrap the string around the bolt threads or shoulder. Knead, work, mash & twist the butyl cones into the threads of the bolt and shape them like a cone.
Step 5 – Apply a thin layer of marine butyl tape to the rest of the base. It is perfectly ok to stretch and pull on the butyl while laying it on in order to make slightly thinner. The tape I use, Bed-It Tape, is 1/2″ wide by 1/16″ thick and is specifically formulated for sealing deck hardware.
Step 6- In colder climates you may want to warm the butyl to soften it some. After warming, with heat gun on the warm setting, or a heat lamp, line up the bolts with the holes and press the hardware firmly onto the deck. I will often stand on the fitting, if possible, to partially displace and seat the butyl.
Step 7 – Install the backing plates, in this case I just used fender washers, and begin to tighten the fitting.
Please use suitable backing plates. These fender washers would not be the best choice in a stanchion or deck cleat as they are far to thin and a little on the small side, diameter wise, for a cored deck.
You will be best to have two people or a good way to keep the bolt from moving.
Ideally you do not want the machine screw or bolt to spin, but if you absolutely have to, you can slightly soften the butyl with a q-tip and some mineral spirits and go for it.
Step 8 – Peel away excess
ooze and clean with Meguiars cleaner wax, or a rag slightly dampened with mineral spirits. Do not saturate the rag with mineral spirits as it can creep under the fitting, just damp.