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-   -   Newbie glassing over a thru-hull (http://www.sailnet.com/forums/gear-maintenance/52164-newbie-glassing-over-thru-hull.html)

squishface 03-05-2009 03:56 PM

Newbie glassing over a thru-hull
 
Greetings to all. I want to fill in 2 un-used corroded through-hulls in my boat (fiberglass), It's my first time doing fiberglassing and I've been reading 'hull and deck repair' by don casey (any other good books on it you guys can recommend?), but have a few questions I would like to ask. They're both below the waterline and about 3-4 inches diameter. I don't care much for looks, just strength.

The idea then, is that I have some kind of plastic backing plate on the outside of the hull, put a bit of gelcoat down first, then glass on top of that.

I would like to use epoxy if possible, just because the book said its stronger. Can I still use epoxy if I'm putting a layer of gelcoat down first, or should I just use polyester resin? Or can I use something other than gelcoat for the outside layer, like just pure epoxy or paint?

And secondly, how do I stop the epoxy/resin/gelcoat bonding to the plastic backing plate?

Thanks in advance

sailingdog 03-05-2009 04:12 PM

Squishface—

You would want to remove the through hulls from the hull. Then you would want to grind the fiberglass to a 12:1 bevel for the thickness of the laminate in the area of the repair. This means that if the fiberglass is 1/2" thick, you would grind a beveled area 6" in radius from the through-hull's original location. Be sure to wash the area well with a good solvent wash like Fiberglass Prep Wash 202 before doing the grinding, since you don't want to grind any oil or wax into the fiberglass.

Then you would wet the area with straight epoxy, and the laminate it with several layers of fiberglass cloth or roving, depending on the thickness of the laminate you need to build up. There are two schools of thought on this. You can start with a small patch and then use increasingly larger patches, or you can start with a large patch and then use increasingly smaller patches. Either will work. The patches should start about 2-3" wider than the actual hole and end up as large as the beveled area, with about 3-4" size difference in diameter between the patches or so. For a 3-4" hole, I'd recommend glassing it over from both sides if you can, but if not, use a backing board on the interior to help keep the patches smoothly integrated to the hull.

Wet each layer out with the minimum epoxy to get it fully wetted out. Having a high-glass to resin ratio will provide the strongest repair.

Once the hole is patched. You will want to use thickened epoxy to fill and fair it so the hull is smooth. Chopped glass fibers or Microballons are both good thickeners to use, but you do have to give the thickened epoxy a final skim coat of unthickened epoxy to seal the microballons after sanding it fair if you use them.

It will be easier and simpler if you use a no-blush epoxy, like MAS epoxy.

The West System's website has a lot of good articles on this type of work, and they have a newsletter you can sign up for as well.

squishface 03-05-2009 08:03 PM

Thanks for your reply sailingdog. Will the epoxy bond to the backing plate? How do I prevent that?

okawbow 03-05-2009 08:32 PM

I have used a sheet of wax paper taped to the backing plate with good results. You can also put a couple coats of paste wax on surfaces you want to release. I also just finished a repair to a keel, where I spread thickened epoxy over the glass build-up, and then taped a piece of wax paper over the wet epoxy. I then used a plastic speader and a flat piece of wood to flatten and smooth the wax paper covered epoxy. When the epoxy had firmed up, I peeled the waxpaper, and had a smooth, flat surface that needed very little sanding and fairing.

sailingdog 03-05-2009 08:42 PM

You can use waxed paper as Okabow suggests, or you can wax the surface of the plate or spray it with mold release compound.
Quote:

Originally Posted by squishface (Post 458583)
Thanks for your reply sailingdog. Will the epoxy bond to the backing plate? How do I prevent that?


mattstamour 03-06-2009 10:54 AM

My Catalina 25 had the old style unsafe gatevalves on the thru hulls for the sinks. I closed the gate valves, then filled the holes from the outside of the hull with marinetex. The holes were less than an inch in diameter.

Is that unsafe?

sailingfool 03-06-2009 11:24 AM

Quote:

Originally Posted by mattstamour (Post 458781)
My Catalina 25 had the old style unsafe gatevalves on the thru hulls for the sinks. I closed the gate valves, then filled the holes from the outside of the hull with marinetex. The holes were less than an inch in diameter.

Is that unsafe?

I would say so. The marine tex inside the through hull is likely to break up over time due to different freezing/contraction rates or just age. That will again make the gate valves as critical as before...if they are a weak link, which they are, you should get ride of them, either replace with proper seacocks, or remove and properly glass over their openings.

sailingdog 03-06-2009 12:13 PM

The problem with doing that is if the metal through-hull corrodes enough, it may give way and leave you with two 1" or so holes in the bottom of your boat. Removing the through hulls and glassing over them is really the only truly safe repair.
Quote:

Originally Posted by mattstamour (Post 458781)
My Catalina 25 had the old style unsafe gatevalves on the thru hulls for the sinks. I closed the gate valves, then filled the holes from the outside of the hull with marinetex. The holes were less than an inch in diameter.

Is that unsafe?


jonb 03-06-2009 11:25 PM

Glassing over a small thru-hull
 
I have a need to fill in a 3/4" hole remaining after removal of a thru-hull.
Would the technique for a filling a smaller hole still require a 1/12 taper on the outside of the hull?
I was planning to use multiple layers of glass cloth on the inside of the hull where there is a recess for the thru hull (where there is no core). I was thinking of using milled glass fibers to fill the hole itself and have a very small taper of maybe only 1" around hole again filled with the milled fibers.

Is this sufficient or should I be using glass cloth on the outside of the hull with a larger tapered area.

sailingdog 03-07-2009 12:07 AM

Quote:

Originally Posted by jonb (Post 459144)
I have a need to fill in a 3/4" hole remaining after removal of a thru-hull.
Would the technique for a filling a smaller hole still require a 1/12 taper on the outside of the hull?
I was planning to use multiple layers of glass cloth on the inside of the hull where there is a recess for the thru hull (where there is no core). I was thinking of using milled glass fibers to fill the hole itself and have a very small taper of maybe only 1" around hole again filled with the milled fibers.

Is this sufficient or should I be using glass cloth on the outside of the hull with a larger tapered area.

Provided you grind the surface to get the glass on the interior to bond properly, I don't see a problem with doing it this way. You're basically patching it from both sides, but I'd recommend putting a few layers of glass on the outside just to be safe. :)


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