Thru-Hull Patching
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I had six thru-hull holes to patch on the bottom. I used a spring loaded curtain rod and a brace to hold a piece of masonite with some plastic sheeting on it's face against the hull over the patch. This gave a very nice surface to the patch that required less fairing to get flush. The two horizontal boards of the brace held the masonite so it conformed to the shape of the hull. I later switched to a piece of plexiglass which gave a better surface and was easier to handle. I spread some wax on the surface of the plexi to ensure it would not bond to the epoxy.

The photos here show the patched thru-hull that had the OEM installed speedo impeller under the access panel at the bottom of the companionway ladder (about as bad a spot for the impeller as you can get - behind the keel). The outside is a little wrinkly because I used a piece of polyethylene sheet on top of my patching panel instead of plexi and it had some folds in it. The plexi works better and gives a smoother surface but you do have to wax it.

I followed the proceedure in the West System manual for patching a hole like this. I ground the surface aroung the hole to a 12:1 bevel on both sides. I cut circles of fiberglass material (biaxial with mat) in increasing sizes to fill in the bevel and a few to fit in the hole.