I just replaced three
I just repalced three thru hull and and seacocks on my 32-2, both 1.25" cockpit drains and the engine intake, which was .5" and I upsized to .75".
The setup was original thru hulls to 90 degree street els to ball valves. The original wood backing plates were damp, and some of the bronze fittings looked reddish and calcified. I thought after 43 years (at least for the thru hulls) I'd take this on.
Conflicting views abound: seacock vs. ball valves, 5200 or something less tenacious, wood backing blocks or G10. etc.
Basically I used Mail Sail's tutorial as a guide. I bought the Groco flange adapters. Note that Groco also makes a predrilled backing block, and there is also
this guy on eBay that sells a verson with matching Groco bolt pattern, which I went with. Buying three backing blocks for $95 saved me some time.
FYI West Marine will also now price match Jamestown and Defender which came in handy.
Hauled in the yard I dissassembled the old fittings. Some were fused together, while others were barely connected!
One very surprising discovery was that the backing blocks were set deep into the hull. Once I chipped them out I was left with very uneven surfaces for the new backing blocks, and I was faced with a choice: sand the area flat, or build it up! I decided to mix up epoxy thickened with chopped fiberglass and colloidal silica. I then waxed the thru hulls and bolted them down, using them to sandwich the thickened epoxy between the backing block and the hull. After it cured, our came the thru hulls and the new backing blocks were bonded to the hull.
Should you choose to do this, make sure you thoroughly clean up/rough up the surfaces, and don't be stingy when you mix up the thickened epoxy; you want ample material to be between the backing block and the hull. Had I wanted to use 5200 and not thickened expoxy I think I would have had to sand the hull flat, and I didn't want to make the hull thinner.
I then make bronze studs from 5/16" bolts, and dry fit the entiire operation.
Once I was ready to install I used Sikaflex 291, mainly becuase not having make this repair before I wanted to make sure I would have been able to get the fittings apart; now that they are in and do not leak, I wish I would have used 5200 AS I NEVER HOPE TO TOUCH THIS AGAIN.