How to reduce the diameter of a thru hull to install new skin fitting?

[Icarus]

Clean the oversized through hole, use some acetone then bed in the new fitting with an epoxy/fibre filler...West System or similar.
I am a shipwright and we bond all underwater skin fittings with epoxy..

[nrx500]

The 3mm air space around the replacement fitting's threaded body will be covered by the flange on the outside and the backing plate on the inside.

Prepare the hole by sanding it clean on the inside and outside. Now cover the replacement fitting with wax paper on the threaded body of the shaft and the flange. Cut a stick long enough to jam between the ground and the replacement fitting in the hull. Now mix up a batch of West System G Flex Thickened Epoxy.

Put the epoxy on the wax paper covering the flange and threaded shaft. Push the fitting into the hole and jam it in place with the stick. Wipe off the excess epoxy that is pushed out from between the flange and hull.

Go inside the boat and make sure the gap between the fitting and hull is filled with epoxy. Trowel the excess epoxy even with the inside of the hull. Let the epoxy cure.

Remove the fitting and peel off the wax paper. Get a piece of quarter inch thick G-10 fiberglass backing board https://www.jamestowndistributors.co...ct-detail/8758 and use your circle saw to drill a hole in the middle for the new fitting. Trim the outside of the G-10 board to give a comfortable margin around the outside of the fitting. Epoxy the G-10 backing plate to the hull, making sure the hole perfectly aligns with the hole in the boat. You could use the fitting to line them up, but put fresh wax paper on the threads to keep the epoxy out.

When everything cures, bed the new fitting in place and screw it in. There is no way this repair is going to leak.

[captnknopf]

How about going with the next bigger thruhull fitting and a reducer or bigger seacock and reducing it after the seacock. Not much work and the cost of the next bigger fitting may be less than the cost of the glassing and drilling etc.

[Lee Jerry]

Generally some good advice can be found above, but I'll add a couple ideas I didn't see. To be used either separately or more likely combined.

First, instead of covering across the entire hole in glass, just wrap short pieces of tape around the perimeter, maybe eight pieces every 45 deg. As you said, you'd be cutting it away anyway.

For filling the gap, of course thickened epoxy is an option. But there is also West system six10. It's pretty easy / convenient. (But maybe not thick enough to cross gaps quite that big; see above.)