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  #1  
Old 10-15-2009
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How to finish bottom patch?

I'd like to patch over an old below the water line through-hull. Actually, I rather not, but I've been advised that the insurance company might expect me to. I'm not sure why a capped off through-hull is unsafe but once you stick a seacock and hose on it, clearly adding more points of failure while removing none, it's ok.

I was planning to use system 3 epoxy and 1708 (too heavy?) biaxial cloth with mat. I was also planning on doing the patching from the inside since I have easy access and there will be a lot less fairing and painting to do that way. Also easier to keep the glass in place with gravity working for me instead of against me. It's a small through-hull, probably 1". On the outside I was planning to fill with epoxy thickened with phenolic microballons (e.g. west 407) and then fair.

What, if anything, do I put on after fairing and before bottom paint? I was planning on using gel coat, but I've since found that polyester gel coat does not adhere to epoxy very well. Should I just use nothing before the bottom paint? Or just paint on some unthicked epoxy over the faired epoxy?
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Tap there are many threads here on this, do a search using google and the key word epoxy and thru hull

also west systems has videos on you tube about this too

yes the biaxial is too heavy 7 oz glass is perfect, and the best is to do the repair from both sides. also gelcoat will stick just fine to sanded epoxy for what you are doing.

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You really should patch it from BOTH sides...as that will create a much stronger fix than patching from just one.

You don't need to prime the epoxy, but can prime it with something like Interprotect 2000E barrier coat.

The reason the capped off seacock is seen as a danger is the cap may deteriorate over time, for instance, a bronze one may de-zincify and fail... and that would lead to the boat sinking.
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I've painted extensive bottom areas what were faired with epoxy/microballoon mix, just sand and paint. The paint held the same as other parts of the bottoms. Gelcoat is a waste of time and materials.
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Old 10-16-2009
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I agree with the dog - both sides is the only good way to do it. Grind a taper on the outside only, probably 3/4" larger than the hole. Tape the inside with duct tape and do the outside first. 1708 is good as it's strong and has some resistance to wrinkling which thin cloth does not. Lay the larger precut circle in first and go progressively smaller. This way when you sand you won't go through the strongest piece. After fairing with thickened epoxy coat with straight epoxy and wash blush off before bottom paint. Apply 2 or 3 coats to ensure the surface is impermeable to water. Barrier coat isn't necessary as the epoxy is the barrier. Gelcoat is unnecessary and is water permeable which epoxy is not. I'd mix the fairing coat with colloidal silica which is tougher to sand, but won't absorb water like sanded microballoons will. If you're careful with the fairing coat and it's mixed to a peanut butter consistency not much sanding will be needed. On the inside sand the area an inch or so larger all around than the hole. Fill any dip where the hole was with thickened epoxy and lay a couple layers of cloth over the area wet out with straight epoxy, the smallest in size first, then the larger. Make sure there are no rough spots after it's set and coat with straight epoxy. No fairing required inside as cosmetica aren't an issue in the bilge.
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I've searched quite a bit already, my plan so far didn't come out of thin air after all, and watched all the videos I could find on youtube. They never say exactly what glass to use for a repair, though what I found about glass types seemed to indicate that stitched biaxial would be stronger than just cloth. Also that mat has better adhesion than woven fabric, so boat hulls are usually made with alternating layers of woven roving and mat. So stitched biaxial with mat backing seemed like the right choice. I don't see anyone selling 7oz fabric, mostly just 4, 6 and 10. I take it four layers of 6 oz fabric should be about as thick as one layer of 1708?

West's boat repair manual, section 2.2, says "these finishes are recommended for above the waterline only. Use west system for all repairs and coatings below the waterline, prior to applying bottom paint." It's not entirely clear to me if the warning about below the waterline use applies only to polyurethane paints or if it includes polyester gel coat as well. The section on repairing keels and rudders says to apply several coats of epoxy after fairing, then wet sand, then bottom paint. No gel coat or barrier coat.

Evercoat's gel coat information says quite clearly on the first page, "IMPORTANT: GEL COAT SHOULD NOT BE APPLIED OVER AN EPOXY."

So the gel coat maker says you can't put gel coat over epoxy and the epoxy maker says you can (but maybe not below the water line). That really clears things up. My searching found a lot more people saying not to put gel coat over epoxy than the other way around.

This west video says after fairing, coat with two coats of epoxy with a white pigment added. Of course it doesn't go into any detail about what happens after you're done using west products. Maybe you go direct to bottom paint or maybe there is something else? This west video just says to paint or apply gel coat after the last glass layer is dry. No fairing even. This other west video says to finish with six coats of epoxy combined with 422 barrier coating additive (aluminum & mica powder), then bottom paint. I'm pretty sure I've seen videos involving gel coat after fairing too.

I haven't see any guides describe patching from both sides. It's always just one, though I have found discussions about which side is better. Any links to a patch from both side guide? I've looked at tons of them and don't recall any, yet now two people suggest it.

Both sides has an appeal, as half the hull thickness to taper will give a patch with half the radius, which is just one quarter the area. Two 1/4 area patches is still just half the total patch area vs one side patching. The problem is that any grinding on the outside of the boat is very costly. Full tent, vacuum sander rental, etc. Hundreds of dollars. Assuming the boatyard will even allow it in the first place. Plus I have a bigger spot on the outside to fair and paint.
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Quote:
Originally Posted by sailingdog View Post
The reason the capped off seacock is seen as a danger is the cap may deteriorate over time, for instance, a bronze one may de-zincify and fail... and that would lead to the boat sinking.
Thanks for the explanation. Still, it seems like the rubber hose for an open cockpit drain is much weaker than a bronze cap. I'm seriously considering putting a new through-hull and seacock on if I can get away with that. Much cheaper and should only take a fraction of the time. If a capped seacock is also too dangerous, maybe I need another cockpit drain somewhere, or a raw water inlet for the galley sink, I don't think the boat has one, or a fish finder transducer, I think the hole is near the center line... all much cheaper than patching over the hole.
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Gelcoat is NOT necessary underwater. It's because of gelcoat that people have spent thousands in some cases and countless hours barrier coating their gelcoat with epoxy. You don't even need the white pigment. After fairing 2 or 3 coats of straight epoxy is all the barrier coat you need. Wet sand and any bottom paint will adhere. By the way you can apply gelcoat to epoxy as outlined by West but not underwater. When you do the inside there is no reason to grind a taper as you're not trying to make the surface perfect - that is only for the outside and a little raised section of the bilge is not important. If you want to learn about patching both sides go to The Plastic Classic Forum • Index page
or Northern Yacht Restoration | Tim Lackey:* One Man, One Boat at a Time where every job is documented step by step with pictures.
Brian
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7 oz glass is what the packs of glass from west marine are, they are a good middle of the road. the problem with the thicker stuff is getting it properly saturated with out having too much resin. yes thicker is stronger until its not totally wetted out, or too much resin.

you are right about the out side fairing and sanding issues. one way around is to just do the tappering on the side, then 2 layers of thinner glass on the out side, to "lock" the inside repair in place. just some minor sanding out side before and after glassing is all it should need
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But you have to grind and sand fair outside no matter how you do it, so might as well do it right.
Brian
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