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Old 20-04-2017, 02:30   #1
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Epoxy on gelcoat and installing thru-hull fittings with epoxy

Hi,

I am about to install new thru-hull fittings. I have chosen Tru-Design composite fittings. Tru-Design recommends installation with West System Epoxy. I browsed the internet for installation tips, started with removing the old ones, and sandpapered down to the GRP hull (removed gelcoat around the holes). Now I am having some thoughts about the application of epoxy and the installation.

We want to apply a epoxy barrier coat on the GRP around and inside the holes and then install the fittings with epoxy. Preparation has been done with 80-sandpaper and cleaning will be done with acetone.

Is sandpaper 80 enough or should I use something rougher or finer?
How many layers should I apply as a barrier coat? At what interval can I apply the layers? To my knowledge the coating layers should be without filler so the epoxy can penetrate the material as good as possible, is this correct?

Will the epoxy bond sufficient to the gelcoat around the GRP to make the hull “waterproof”? The gelcoat has been sanded and will be prepared with cleaning as well. (see picture)

Should I let the barrier coating layers harden before mounting the thru-hulls or can it be done directly to the coating layers before hardening? When mounting the thru-hulls I have previously been recommended to use filler, do you agree?

The boat has not previously been treated with epoxy.

Yours sincerely,
Tobias
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Old 20-04-2017, 05:06   #2
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Re: Epoxy on gelcoat and installing thru-hull fittings with epoxy

Technically you can dispense with the barrier coat altogether. It's just epoxy paint, and it's purpose is to prevent water penetration into the gel. 80 grit is plenty rough enough. If it were me I would install the through hull per their instructions, although I would probably brush on epoxy neat on the hull before topping with filled and installing to ensure the best bond with the GRP. Then barrier coat the surround and up to the flange. It's really up to you, both approaches amount to the same end result.

If you do barrier coat before the install, you want to hot coat your coats, I would do at least four, and then install the thru hull after the last coat has kicked but before it's fully cured. That will ensure a chemical bond between all the layers and the bedding epoxy as well.
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Old 20-04-2017, 05:50   #3
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Re: Epoxy on gelcoat and installing thru-hull fittings with epoxy

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Technically you can dispense with the barrier coat altogether. It's just epoxy paint, and it's purpose is to prevent water penetration into the gel. 80 grit is plenty rough enough. If it were me I would install the through hull per their instructions, although I would probably brush on epoxy neat on the hull before topping with filled and installing to ensure the best bond with the GRP. Then barrier coat the surround and up to the flange. It's really up to you, both approaches amount to the same end result.

If you do barrier coat before the install, you want to hot coat your coats, I would do at least four, and then install the thru hull after the last coat has kicked but before it's fully cured. That will ensure a chemical bond between all the layers and the bedding epoxy as well.
Thank you for your answer. Your approach sounds smarter for achieving a strong bond between the thru-hull and the GRP.

Just to be clear, in your approach the steps are:
1. Prepare the surface (80 grit and acetone)
2. Brush on epoxy (without filler) on the hull.
3. Install the thru-hull with epoxy + filler. Would you wait for the first brushed layer to “kick”? Would you apply epoxy with filler on top of the first layer or only on the thru-hull?
4. Apply barrier coat after the first part/s has “kicked” (filler or no filler??) in layers up to the flange. With hot coating technique with "kick" in between layers. How many layers?
5. And this should seal against the gelcoat as well?

I am using West System Epoxy resin 105 with fast hardener 205 (I am from the cold country called Sweden…).
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Old 20-04-2017, 06:07   #4
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Re: Epoxy on gelcoat and installing thru-hull fittings with epoxy

suijin knows what he's talking about..Listen to him...


4 layers is good...kick means the epoxy no longer flows, but has not yet become a solid. Its the gummy or green stage.


By having those 4 hot coats, you are creating a nice soft bed for the thru-hull to sink into and become very well adhered to the hull when it cures.
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Old 20-04-2017, 07:15   #5
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Re: Epoxy on gelcoat and installing thru-hull fittings with epoxy

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suijin knows what he's talking about..Listen to him...


4 layers is good...kick means the epoxy no longer flows, but has not yet become a solid. Its the gummy or green stage.


By having those 4 hot coats, you are creating a nice soft bed for the thru-hull to sink into and become very well adhered to the hull when it cures.
Thank you for your answer, nice that you agree with Suijin.

Should I let the first layer kick, then apply the next layer, let it kick and so on? Should these layers have any filler?

West system says "Ensure that the previous coat is still tacky but has also cured firmly enough to support the weight of the next coat.", which I understand as; it should kick before adding the next layer. Will I manage to fit the thru-full fitting before the first layer has reach solid cure with this approach?

With this approach I think I will add two more layers after the final cure (sandpaper 80 grit first) with Barrier Coat Additive (422). What do you think? Four layers might be more than enough though...

I will do some testing before I start to fully understand when the epoxy has "kicked".
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Old 20-04-2017, 07:30   #6
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Re: Epoxy on gelcoat and installing thru-hull fittings with epoxy

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Originally Posted by TobiasLager View Post
Thank you for your answer, nice that you agree with Suijin.

Should I let the first layer kick, then apply the next layer, let it kick and so on? Should these layers have any filler?

West system says "Ensure that the previous coat is still tacky but has also cured firmly enough to support the weight of the next coat.", which I understand as; it should kick before adding the next layer. Will I manage to fit the thru-full fitting before the first layer has reach solid cure with this approach?

With this approach I think I will add two more layers after the final cure (sandpaper 80 grit first) with Barrier Coat Additive (422). What do you think? Four layers might be more than enough though...

I will do some testing before I start to fully understand when the epoxy has "kicked".
Kicked means that the epoxy is stiff enough that it's not going to get dragged when you try to brush on the next coat. If it does drag, stop...let it cure and sand it back down again.

if you can press your finger into the epoxy and your finger doesn't get wet, but it DOES leave a finger print behind, then it's green. You don't have to wait this long, but you do need to wait long enough for it to be hard enough not to get dragged like I said by the next coat. It takes some practice...it won't always be the same amount of time...the ambient temperature and the thickness of each layer will totally affect how long it takes for the epoxy to kick as well as how long the epoxy as been allowed to ripen in the cup after you mix it. You might also want to mix a small amount of epoxy and leave it in the cup just to see what happens. It's a waste of epoxy, but at least you can see how epoxy cures in faster motion and what happens to it at each stage. If your portion is large enough it will get hot enough to start a fire. I don't recommend doing this in-pot test with more than 2 pumps of west system resin and hardener. 3 pumps may be enough to start the fire.

Testing on a spare piece of plywood just to see what and how the epoxy feels like at each stage is a very good idea. Apply it then come back and check it once each hour. Press your finger into it, slide your finger through it, see what happens when it skins over, but is still liquid inside...etc.etc.
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Old 20-04-2017, 08:57   #7
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Re: Epoxy on gelcoat and installing thru-hull fittings with epoxy

I recently had True-Design valves fitted. The engineer used Sikaflex 291i rather than epoxy. Does anyone see any issues with this?
Unfortunately I did not instruct him to route our some balsa core and fill with epoxy, so I am hoping the Sikafex did a good job of sealing the balsa core.
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Old 20-04-2017, 09:46   #8
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Re: Epoxy on gelcoat and installing thru-hull fittings with epoxy

Not really an issue, I would just keep an eye on your thru-hulls and inspect them regularly as you should no matter how you bed them. Haulouts are an important part of the inspection process and your schedule may vary depending on your confidence in your boat. You'll have a better idea of how soon you should haul out by diving on your boat and seeing how things look down there.
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Old 20-04-2017, 10:03   #9
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Re: Epoxy on gelcoat and installing thru-hull fittings with epoxy

Quote:
Originally Posted by TobiasLager View Post
I am about to install new thru-hull fittings. I have chosen Tru-Design composite fittings. Tru-Design recommends installation with West System Epoxy.
Seeking clarification - is Tru-Design recommending that one epoxies their thru-hulls into place - eg epoxy is what you goop the fitting with before you place it and tighten up the mounting nut?

I'm used to making a hole, gouging back the hull core a little if present and filling that in with epoxy (wet epoxy, then epoxy with filler), epoxying in backing plate if required, epoxy-filling any voids, grinding marks etc... final sanding, then using the appropriate sealant (like 4200 or Sikaflex) when installing the thruhull.
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Old 20-04-2017, 10:14   #10
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Re: Epoxy on gelcoat and installing thru-hull fittings with epoxy

If you do accidentally need to let a layer fully cure (like if the expoy runs or drags as suggested above) -- then wipe the cured layer with acetone before and after sanding. This will help remove any waxy "amine blush" that might appear during the cure. If there is a blush and you don't remove it then you end up with a less-than-ideal bond between the layers.
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Old 20-04-2017, 10:25   #11
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Re: Epoxy on gelcoat and installing thru-hull fittings with epoxy

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Originally Posted by Suijin View Post

(...) then install the thru hull after the last coat has kicked but before it's fully cured. That will ensure a chemical bond between all the layers and the bedding epoxy as well.
I do this differently. I let it cure then bed the thru hull with a sealant.

Are the two methods equivalent? Where is the catch?

THX,
b.
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Old 20-04-2017, 10:50   #12
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Re: Epoxy on gelcoat and installing thru-hull fittings with epoxy

So to my eye, the difference is the future.


With bedding compound or sealant you are hoping when the time comes years down the line that you can remove the thruhull without having to grind it out.


When you use epoxy, you realize you will probably have to grind it out anyway no matter what you do, so why not just make it all that much more secure.


that's how my eye sees it anyway.
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Old 20-04-2017, 11:44   #13
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Re: Epoxy on gelcoat and installing thru-hull fittings with epoxy

Seems to me like you went way overboard. If your fitting is has a flange that is a good bit bigger than the hole and the hole goes through solid and there is not a balsa or plywood core, then just cut hole and seal it on both sides with butyl tape and screw it tight and then use a dull knife to scrape the butyl tape off the fitting.
If you are going through a core then you need to drill a hole about 1/4 to 1/2 inch bigger than the hole, fill the whole thing with epoxy(digging out the core about 1/8, and then re-drill the hole). But it doesn't look like that's what you have.
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Old 20-04-2017, 12:09   #14
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Re: Epoxy on gelcoat and installing thru-hull fittings with epoxy

I am very surprised by this recommendation to put thru-hulls with epoxy !
For me, thru-hulls are always fitted with PU or MS polymere seal.
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Old 20-04-2017, 12:43   #15
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Re: Epoxy on gelcoat and installing thru-hull fittings with epoxy

Suijin,
Please explain the"hot coat" process.
Thank you, Ken
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