Originally Posted by Noreastern
I am planning on patching (glassing in) seven thru-hulls. All are about two-two and half inch diameter holes in hull. My question is, better to patch from outside or inside the hull?
I think patching from inside would make the job of fairing outside hull much easier since grinding a taper 12 to one (around 12 inch diameter) on the inside keeping the hole outside 2 inches and also working with gravity.
Down fall to patching inside I believe would be taper of "plug" would be stronger from outside.
Appreciate any thoughts!
I must have done a thousand of these. You are right, fairing would be easier if done from the inside. But it's a better repair done from the outside. And for me the key issue-this is a very messy process and it's much easier to deal with the grinding dust when outside the boat, not to mention the mess of the layup
. Once you've dusted out a few interiors and realize the level of masking required to prevent this, it's a no-brainer. This should be a very quick job.
You say 12-1 gives a 12" diameter. I assume this means the hull is 1" approximately in way of the through hull and you intend to back grind until almost through in the center. I have seen many people do it this way and I think it is overkill. In fact I think making a repair that large for a through hull is potentially making a weak spot by making an unnecessarily large repair. Hole saw a piece of flat stock fiberglass
1/2" thick to the appropriate diameter and bond it in place flush with the inside first, using hot glue and some small wood blocks for perfect alignment to the interior
surface. Then back grind the exterior 6-8", grinding until you hit the disc you bonded in and have a scratch on it. Then glass it up and fair it, barrier coat it, and bottom paint
it. Usually I slap a glass patch on the inside too. It only needs to be a few matts. I'd do it all in poly. This shouldn't take more than an afternoon, aside from the coating. Which will only take a short time aside from waiting for paint
to dry. This method is more than strong enough.