This photo shows a side view of the assembly. The hull & sealant are the only pieces missing.

First you need to de-wax the backing plate. Polyester finishing resin has added paraffin to aid in the cure process. This wax, and the wax coating from what you laid it up on, must be removed prior to installation or nothing will adhere well.

Over the years I have found the dedicated, but overly priced, cocktails of chemicals sold by Pettit or Interlux as de-waxers definitely work. I only wish I could figure out what the chemical mix is because I bet I could mix it up for considerably less than what they sell it for.

Before you begin sanding the hull you should remove any surface wax. Gelcoat, like polyester finishing resin, is also an air inhibited cure so any gelcoat on the interior of a hull has a waxy surface. In order for proper adhesion any interior gelcoat needs to be removed.

Once your dry fit is confirmed parallel, and all sides of the mushroom head sit flush on the hulls exterior you will need to do one more round of de-waxing of both hull and backing block.

After that is done you need to wax both the flange (not the backer block) and the thru-hull with copious amounts of mold release wax, butchers wax or even Collinite. The wax should be a paste type carnuba. After you have waxed it cut a scrap board to length and wedge the thru-hull into place. If you are on gravel or dirt lay a board on the ground to wedge the upright board against.

Contrary to popular belief you really do not need two people to install a thru-hull if you can be a little creative.

For illustration I choose to make a formula to show the consistency better. Whether epoxy, polyester or vinylester all should be plenty strong for this application. Epoxy is considerably stronger than polyester resin, in a secondary bond, and is what I personally use. My mix for illustration consisted of chopped strand fibers, West System 406 Coloidal silica or Cabosil, some West 404 and polyester resin. The same mix can be made with epoxy. When I shot this I was not ready to lay it up so used the less expensive polyester resin.

I like making my own "recipe" because it gives me more control over consistency and how it fillets. Kitty hair is another product I sometimes use but it will not fillet as well as your own custom mix will. For catalyst in the illustration I used some red dyed MEK-P. I like dyed MEK-P so you know you have it thoroughly mixed.

Wear rubber gloves and apply a thin coat of the mix to both the hull and backing block, then slather a large amount onto the backing block and thread it onto the thru-hull about three threads. I then used vise grips to grip the flange to prevent it from spinning and went out side the boat and used my step wrench to tighten down the thru-hull and seat the backing block.

Note: Epoxy will yield a much stronger bond than polyester resin but even polyester should give you about 500 PSI adhesion. ABYC standards call for the seacock to support a 500 pound static load for 30 seconds at the inner most hard fitting or hose barb.