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post #1 of 14 Old 11-25-2009 Thread Starter
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Fiberglass repair question

I have a piece of plexi over a hole in my cockpit where an insert used to be. The autohelm has its mounts there and it is currently cracked and has a tendency to leak so its taped over with blue tape at the moment.

My plan is to fiberglass over the hole to seal it up and then remount the autohelm bracket to the fiberglass. I have decent access to the inside of the hole so was planning to put a layer of mat over the inside of the hole and then from the outside build up layers until it is to the level of the gelcoat and then put a layer of gelcoat on it. Most repairs seem to recoment going the other way and putting a backing on the outside and doing the gelcoat first.

The fiberglass is not very thick somewhere around 3/8ths I would guess so it would only take a few layers to build it up.

Is that a reasonable way to go about it?

here is a pic of the plexi I want to get rid of:

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post #2 of 14 Old 11-25-2009
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You will need to grind the existing fiberglass to about a 12:1 taper (or for about 4.2 inches around the opening and then glass over the whole area. The reason for this is to not create a hinge point or weak spot at the edge of the repaired area. Simply backing the area with a layer of matt and then glassing in the hole will leave a big section that could easily get punched out by a wave pooping the boat. Once you've got the surface even with the rest, you can sand it and then gelcoat over it.

I would highly recommend using epoxy resin for this repair, rather than polyester or vinylester resin. This kind of repair relies mostly on the secondary bonding characteristics, and epoxy resin has the strongest adhesive or secondary bonding characteristics of the three. It is also simpler to work with and more forgiving in many ways. I'd recommend buying your epoxy from Progressive Epoxy Polymers in NH. Progressive Epoxy Polymers and Resins - Home Page Pittsfield, NH is their website.

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post #3 of 14 Old 11-25-2009
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Everything that dog said. Plus mat by itself has no great strength. Best to use a combination of mat and thicker cloth - roving or ideally biaxial 1708 which is 17 oz stitched roving (not woven) with the mat already on one side. It`s strong and builds up thickness quite fast. It`s fully compatible with epoxy as well - some types of mat by themselves are not compatible with epoxy. I`d grind the edge like dog suggested and tape a backer to the inside and do all the glasswork from the outside - it`s a lot easier to do that in a small space. After the outside is glassed I would sand the interior and put one layer on but cosmetics aren't an issue there so this should be easy.
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post #4 of 14 Old 11-25-2009 Thread Starter
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I was under the impression that gelcoat would not adhere to an epoxy. I would rather use epoxy for the ease you mention but am I wrong or is one of those times where really good adhesion does not matter?

I get what you say about the force of something hitting it from the outside though. So I see why would want it to act like a plug and not be able to push through the hole you are trying to patch.

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post #5 of 14 Old 11-25-2009
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gelcoat will stick to sanded and cleaned epoxy well enough for what you are doing. in this case its visual and to protect the epoxy from UV. just do all your fairing before you go for gelcoat.

use the west marine gelcoat paste, its white and says non sag, even then you may/will need tint it and it will sag if you try to go too thick. use a foam roller it roll on a few coats until it completely hides the epoxy. after each coat you need to dewax with what ever you have that will work. then wet sand smooth with 220, then try a coat of gelcoat that you tint. if you got it right sand smooth and buff. if not try another coat tinted slightly different, repeat until it matches. each coat should hide the edge of the previous one, you might wind up going an extra few inches past the epoxy. this is fine as it will help hide the repar

my experience is if you dont get it right, as long as its close when you sand it out and feather the edges then buff all most nobody will see it
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post #6 of 14 Old 11-26-2009
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Here's the link for West Epoxy's Fibreglass Repair manual. Gelcoat info is on page 7 but I'd download the whole manual as it's full of good info you might need for the rest of the repair. http://www.westsystem.com/ss/assets/...aintenance.pdf
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post #7 of 14 Old 11-28-2009 Thread Starter
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Hrrm. Looking at it I don't really have the needed space to taper it out on the top and bottom. I will have to clean out the quarter berth and get up in there to measure but I am planning on putting a rack up there on the inside to hang things on. I may try to plan that project around this one and put something over it to add some strength and give me something to mount the hangers on.

Boat projects drive me crazy with the way they run together. Not going to be too much longer and I am going to be making gantt charts to identify the critical paths...

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post #8 of 14 Old 11-28-2009
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okay as for the distance to taper, you can run the taper around corners. also you can do half the taper on the out side and half on the inside, its actually the best way.

after looking at the pic again what i would do, is to taper it out as far as you can. then use a piece of g10 as a filler, or even make your own filler board. the filler is just a bunch of glass and resin made up to the thickness you need, then cut to fit the hole nicely. then after glass the out side, then add a layer or two on the inside.

doing this there is a chance it might crack under sail, but thats what the extra glass on the inside is supposed to mitigate
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post #9 of 14 Old 11-28-2009
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Good info thanks alot! I am going to be doing some fiberglass work this coming spring so getting this first hand info really helps. Thanks
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post #10 of 14 Old 11-29-2009
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No way would I try and do that in situ, even with a backing plate. Cut a piece of ready made fibreglass sheet to size, V out the joint and fit. It does not look structural so no need to go to 12 to 1. As you are going to fit something into the insert you could pre cut drill and trial fit the equipement before glassing it is. I might be tempted to fit some kind of trim bezel rather than try and colour match the insert.
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