Whenever you make a hole in your deck, you need to seal it to avoid water intrusion. This is easy enough when you are repairing a hole in the deck with wood or fiberglass, but what about when you install hardware through the deck?
This is where bedding compound comes into play! Bedding compound will seal up any voids that may exist between the item and the deck. This seals out water and moisture, while adhering the item to the deck. Bedding compound also needs to be forgiving, allowing it to stretch and wiggle as the boat twists through the waves and expands during the heat of the day.
My preferred bedding compound for marine hardware is 4200, manufactured by 3M. While not as popular as 5200, 4200 does an excellent job and is much more forgiving. 5200 is considered a "permanent" adhesive. When 3M (a company famous for making things that stick) says its product is permanent, they mean it! I have seem the top layer of a deck be ripped up with deck hardware that was bedded with 5200.
4200, on the other hand is just a hare's breath less "permanent" than 5200. This means that it will still seal out all traces of water and firmly adhere the fitting to the deck, but it can be removed with enough persuasion in the future.
I like retrieve-ability in everything I do. The thought that something is now permanently installed and can never be removed irks me! Whether it be an implant crown that I place in my dental office or a chainplate, I want to have a method to remove it should the situation arise.
When you use bedding compound, the concept is simple: You want to fill all the voids so that it occupies all the space between the fitting and the deck. To ensure that you have enough compound in there, you need it to squeeze out. Bedding compound is very gooey and is rather hard to clean up; but if you follow a few easy steps, cleanup could be much easier.
In this example, we will be bedding chainplates. The first step is to cover everything you don't want bedding on with tape. Then trim off all the excess tape with a sharp knife.