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Old 25-03-2020, 10:38   #1
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Glassing Over Closely Spaced Through Hulls

Hey again,

Just wondering if any of you have had to glass over multiple holes below the waterline in the same area. I'm removing my toilet intake and output seacocks/through hulls and the adjacent holes (two quarter inch holes) for the speed through the water sensor. These are all close together (about 8 inches apart for the through hulls and 4 for the smaller holes). Normally, one bevels the inside and outside of the hull around the holes and lays mat and epoxy, right? But that doesn't seem Ideal in this case so there must be a better way. I cringe at the thought of making the 4 holes into a larger hole but that's all I could come up with. Surely some of you have had to do a modification like this...
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Old 25-03-2020, 10:48   #2
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Re: Glassing Over Closely Spaced Through Hulls

Typically you don't use mat with epoxy. It is stiff and the binder only dissolves in polyester resin. 1708 biax is probably a better choice.


As for the bevel, I think this is a case for compromise. You've got the idea.
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Old 25-03-2020, 10:56   #3
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Re: Glassing Over Closely Spaced Through Hulls

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Typically you don't use mat with epoxy. It is stiff and the binder only dissolves in polyester resin. 1708 biax is probably a better choice.


As for the bevel, I think this is a case for compromise. You've got the idea.
So biax, West Systems and shallower bevels, you think?
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Old 25-03-2020, 11:05   #4
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Re: Glassing Over Closely Spaced Through Hulls

personally I would just taper out the small holes maybe to a 1" diameter (for a 1/4-3/8 ex-bolt hole) ... both sides. Once it hardens up it's a "plug" that cant come out anyway. Let's face it, it's better than a corroded bolt and sealant that was i n there in the first place.
I understand very wide tapering for repairing a hull large area, but plugging holes I don't feel it's necessary.
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Old 25-03-2020, 11:55   #5
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Re: Glassing Over Closely Spaced Through Hulls

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personally I would just taper out the small holes maybe to a 1" diameter (for a 1/4-3/8 ex-bolt hole) ... both sides. Once it hardens up it's a "plug" that cant come out anyway. Let's face it, it's better than a corroded bolt and sealant that was i n there in the first place.
I understand very wide tapering for repairing a hull large area, but plugging holes I don't feel it's necessary.
Cool. That's true, I'm not sure how it hasn't leaked yet. So you'd just leave unused through hulls in the boat? My fear is they will pop off when I'm not aboard and leave me faced with a long swim home��
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Old 25-03-2020, 12:07   #6
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Re: Glassing Over Closely Spaced Through Hulls

Since they are all close together, nothing stopping you glassing extra layers inside over the whole lot. Just make sure the area is well keyed first.

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Old 25-03-2020, 13:45   #7
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Re: Glassing Over Closely Spaced Through Hulls

I did the same you want to ! I just tapered both inside and outside to 12 to 1 ! 1/2 inch hull thickness = about a 6 inch taper ! I used stitchmat, and unwaxed resin ! Out side first ! 4 layers outside then poly film then semi stiff piece of foam, propped in place with a piece of strapping ! Went inside did the same thing ! I did it in the late winter, so the resin inside had not cured all the way, which allows the inside layup to chemically bond to the outside ! I used un-waxed resin for this reason, that if it cured to fast outside I could still get a chemical bond ! After it kicked I laid a layer mat with waxed resin on both inside and out to allow for sanding ! Surveyor said nice job !
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Old 25-03-2020, 14:08   #8
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Re: Glassing Over Closely Spaced Through Hulls

I normally do something like this:

Remove old through-hulls and clean up crud with a dremel.

Sand the interior and bond a bit of glass plate over the hole.

Grind from the outside until you hit the glass plate. Now you have a decent taper.

Lay pre-cut bits of chop, biax, mat or whatever to build up thickness. Make a little mound.

Grind or sand fair.

Paint with barrier coat.

If the boat is polyester I'd use polyester for a job like this.

I'd keep them separate. Makes for less work and easier fairing.
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Old 25-03-2020, 16:02   #9
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Re: Glassing Over Closely Spaced Through Hulls

How big are the holes and how thick is the hull?

Assuming a solid glass hull, you can grind the outside bevels to halfway deep... so the hull surface that is between the holes... grind it down halfway. The outer perimeter also halfway down and then a 12:1 bevel. Example for 1/2 hull thickness you grind away 1/4 in between the holes and then a bevel from 1/4 deep to nothing over an area 3 wide

After glassing this with epoxy and 1708 and everything is cured: repeat the same on the inside
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Old 25-03-2020, 20:22   #10
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Re: Glassing Over Closely Spaced Through Hulls

I have three unused ex-head through hulls that I would like to glass over. The hull is solid polyester glass.

Ive seen advice above to bevel all the way through from the outside and keep it flat on the inside, versus bevel half way from both outside and inside. Which is correct?

Also, since my hull is polyester, should I use polyester resin or can I use West System epoxy?

To fill the holes, should it be pieces of chopped strand mat flooded with resin or mixed layers of chopped strand mat and cloth or just layers of cloth?
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Old 25-03-2020, 21:32   #11
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Re: Glassing Over Closely Spaced Through Hulls

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Originally Posted by fxykty View Post
I have three unused ex-head through hulls that I would like to glass over. The hull is solid polyester glass.

I’ve seen advice above to bevel all the way through from the outside and keep it flat on the inside, versus bevel half way from both outside and inside. Which is correct?

Also, since my hull is polyester, should I use polyester resin or can I use West System epoxy?

To fill the holes, should it be pieces of chopped strand mat flooded with resin or mixed layers of chopped strand mat and cloth or just layers of cloth?
Grind halfway, doing both sides is easier and stronger, but also more work and sometimes there is no access on the inside. For a cluster of holes this is what you must do or it becomes one big hole. The good thing is that there must be access on the inside because it’s where the seacocks were

Edit: use epoxy, not polyester. Use cloth,biax or 1708, not mat.
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Old 26-03-2020, 01:14   #12
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Re: Glassing Over Closely Spaced Through Hulls

A couple people have said to use no mat with epoxy but suggested using 1708. 1708 IS mat (and biax). It's 17oz biax with a backing layer of 8oz mat. That's where the number 17/08 comes from.

So if you are using epoxy and want to do it without mat, don't use 1708. Use straight biaxial.
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Old 26-03-2020, 07:26   #13
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Re: Glassing Over Closely Spaced Through Hulls

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Originally Posted by Chotu View Post
A couple people have said to use no mat with epoxy but suggested using 1708. 1708 IS mat (and biax). It's 17oz biax with a backing layer of 8oz mat. That's where the number 17/08 comes from.

So if you are using epoxy and want to do it without mat, don't use 1708. Use straight biaxial.
Sorry, that is not the best advice. The reason for not using mat is that the fibers are held together by a binder that is dissolved by polyester but not by epoxy. The 1708 fabric has a mat layer but is stitched instead of using a binder and is perfect for use with epoxy. The matt layer is beneficial for adapting to small surface imperfections so 1708 is preferred over straight biax for this application.
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Old 26-03-2020, 09:55   #14
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Re: Glassing Over Closely Spaced Through Hulls

I save plugs from cutouts previously added.
Some shaping,maybe required.
Add 2 together to get the thickness needed, grind away any gelcoat on them and glass into the holes.
Fair the area, w filler.
Glass over. Barrier coat over the open FRP. In the area.
Done deal!
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Old 26-03-2020, 10:47   #15
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Re: Glassing Over Closely Spaced Through Hulls

I've done precisely that...gelcoated discs glued into same gelcoated holes (area), with reinforcement on the back. But that was on an internal pan or whatnot, and for cosmetic reasons. But I don't think I would do that on the hull or a structural member. Perhaps if the hull thickness were close to the hole diameter, but that's not likely.
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