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Old 07-03-2022, 04:30   #1
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How to plug a screw hole in a foam cored boat?

A PO decided to use screws that are longer than the glass and foam is thick to attach the button snaps to the coach roof, for the exterior window shades. All was fine and dandy until the sealant they used on the screw gave out and I woke up to a dripping sensation on my head.

I went to the local chandlery and picked up some quick setting, flexible sealant and applied a generous helping to the screws and stopped the leaking. I'd like a more permanent fix. One that doesn't involve a hole from the outside to the inside.

I'm guessing the screws were 1/2 inch, so we can assume the total structure is a bit short of that.

I was thinking of mixing up some epoxy and trying to force it into the hole somehow? But I'll need to reinstall the button caps. Am I better off to overdrill holes centered at the through holes, without going all the way through the inside glass, and then laying in epoxy there (maybe some masking tape on the inside to hold the epoxy in?). Once that epoxy sets, get screws that aren't going to poke all the way through and drill into the hardened epoxy?
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Old 07-03-2022, 06:32   #2
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Re: How to plug a screw hole in a foam cored boat?

That will probably work well. The trickiest part is not losing your screw hole location. I would make a template of where the snaps go using some feature on the coach roof. That way once your done filling the overdrilled holes with epoxy, you can relocate the screw holes properly.
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Old 07-03-2022, 06:35   #3
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Re: How to plug a screw hole in a foam cored boat?

I think this is the best way but you have to be very careful to not break the epoxy with the screws. Measure the diameter of the shank and the threads carefully with calipers and find a drill bit that's just in between these numbers. Drill a small pilot hole first and then countersink it before drilling it to size. Then put 3M 4000 on the threads and under the head and screw them in by hand. I'd go with larger diameter but shorter screws to make sure they don't pop through which also means putting tape on the bit so you don't go all the way through the epoxy plug. All this has worked well for me so far and I have a Freedom with a cored deck. You can ream out the core a bit so you have a plug that is sandwiched in by the fiberglass on both sides if you don't overdrill the fiberglass. I use a piece of bent metal in a drill to do this and vacuum out the hole of any debris. Then don't forget to tape the bottom. I use Gorilla tape because I like to tap in there with a pick to make sure all the voids are filled with the epoxy. I've also had the blue tape fail to hold and that's not pretty.
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Old 07-03-2022, 07:00   #4
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Re: How to plug a screw hole in a foam cored boat?

You have the right idea for sure. That is how I have done it.

If you put filler the epoxy doesn't shatter as easy when you screw into it, but the pilot hole is still definitely recommended.

I second the gorilla tape for the bottom.
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Old 07-03-2022, 14:11   #5
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Re: How to plug a screw hole in a foam cored boat?

Quote:
Originally Posted by Muaddib1116 View Post
The trickiest part is not losing your screw hole location.
If I use the current screw hole as the center mark of my over drilled hole, I should be able to put the screw in the middle of the new epoxy plug, right? Or is using the existing hole as the center of thew new plug a bad idea?
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Old 07-03-2022, 14:13   #6
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Re: How to plug a screw hole in a foam cored boat?

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Originally Posted by ttex View Post

If you put filler
Just to make sure I'm on the same page, just some generic filler available from the chandlery will make the epoxy more welcoming of being screwed into, whereas if I use straight epoxy I might crack the new plug when I drill?
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Old 07-03-2022, 14:16   #7
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Re: How to plug a screw hole in a foam cored boat?

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Then put 3M 4000 on the threads and under the head
Is the 3400 to prevent water from wicking it's way down the screw and into the new epoxy plug/potentially into the core?
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Old 07-03-2022, 14:23   #8
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Re: How to plug a screw hole in a foam cored boat?

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Originally Posted by workmaster2n View Post
If I use the current screw hole as the center mark of my over drilled hole, I should be able to put the screw in the middle of the new epoxy plug, right? Or is using the existing hole as the center of thew new plug a bad idea?
If everything goes perfectly, then sure, but I could imagine a few holes drifting as you overdrill or something else. It's up to you.
Quote:
Originally Posted by workmaster2n View Post
Just to make sure I'm on the same page, just some generic filler available from the chandlery will make the epoxy more welcoming of being screwed into, whereas if I use straight epoxy I might crack the new plug when I drill?
I think he means one of a variety of epoxy thickeners that the chandlery will have. Alternatively, gflex epoxy might be fine on it's own, it isn't as hard as most epoxies.
Quote:
Originally Posted by workmaster2n View Post
Is the 3400 to prevent water from wicking it's way down the screw and into the new epoxy plug/potentially into the core?
If you are threading the screws into new holes that are entirely in epoxy, then you don't really need sealant. But it might work as a sort of threadlocker.
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Old 07-03-2022, 14:43   #9
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Re: How to plug a screw hole in a foam cored boat?

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Originally Posted by workmaster2n View Post
If I use the current screw hole as the center mark of my over drilled hole, I should be able to put the screw in the middle of the new epoxy plug, right? Or is using the existing hole as the center of thew new plug a bad idea?
This is the proper way to do it. Yes. Use the existing hole as a pilot hole for your drill bit, and nothing changes.

Drill a hole 4-5 times the diameter of the shank of the screw, using 6mm (1/4") as a minimum size, and the size of the fitting as your maximum size. The bigger the better, as long as it doesn't show.

Be sure to remove all the foam to solid glass on the bottom, use a flat screw driver for this. Do a good job on cleaning the bottom of the hole, and once epoxied, it'll be sealed from any water intrusion.

Fill with thickened epoxy, (use a syringe from West Marine, best option) cotton works well, do not use cabosil for this, as it's fairly brittle. Do not use fairing compound, or fairing filler. West 404 structural filler works well.

Slow cure works better than fast cure, since it takes a little time to fill all the holes, and you don't want it to go off in the cup. Or just mix very small batches with fast cure.

Drill your pilot hole in the centre of the filled area (not hard to get it pretty accurate) and install your screws. Be accurate on your pilot hole size, too big is worse than too small.

I agree with Muaddib1116, sealant is an option at this point.

Pat yourself on the back, you've done a good job.

Cheers.
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Old 07-03-2022, 14:52   #10
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Re: How to plug a screw hole in a foam cored boat?

This^. Drill & Fill.

Its more work the just slopping some sealant on it, but it is a good permanent fix. With a cored structure you will be glad you did in the long run.
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Old 07-03-2022, 17:18   #11
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Re: How to plug a screw hole in a foam cored boat?

https://marinehowto.com Has good detail on how to do this properly so the core is sealed.
Once you have overdrilled the outer skin & reefed out the core (keyhole bits in a Dremel or drill are brilliant for this), A quick but pricier solution for those who (like me) who are lazy, is to use Thixo (Jamestown distributing) injected into the holes & smoothed flush. Next day, drill correct screw core size & chamfer outer frp surface to accept collar of sealant.
I always turn the screw in by hand be patient. if I canít turn it in with moderate force, I keep drilling by 1/32 increments until I can. If you crack the epoxy with too small a drill hole, you have lost the core seal & must start over! The Thixo is less brittle, & less likely to crack than thickened epoxy (which also works fine). If you leave the mixing nozzle in place it will keep for a year or more, just remove old nozzle, screw on a new one, waste an inch or so. Ready to fill again
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Old 07-03-2022, 17:38   #12
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Re: How to plug a screw hole in a foam cored boat?

I've seen a lot of good Ideas on how to resolve it and all will probably work.

How I would do it is a mix. What you have run into is a common problem with foam core boats that can be easily resolved.
Even though the fiberglass has compressive strength the foam core does not and if you do not remove the foam core and fill it with epoxy you will constantly have problems with star cracks in the jell coat around the screw home compression divits and delamination issues down the road. "This also applies to Balsa core in the same way"
You need to clean up the hole through the jell-coat by over drilling it then I would take a bent nail and put it into the hole and spin it around with a drill to enlarge the area between the fiberglass and foam core removing the foam core to make a good seal between the glass on the deck and headliner. this will stop the delamination of the fiberglass between the foam core along with moisture intrusion and the hole to take the load that is required to keep the fastener in place.
Adding filler sounds good but I would use chopped class or carbon fiber over faring compounds in that the fairing compound is nothing but styrene balls "The same material your hull core is made of" that will compromise the strength of your repair when under load.
I would do this for all of the holes you just drilled in that if one fails the others are likely to follow suite for the same reason.
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Old 07-03-2022, 18:08   #13
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Re: How to plug a screw hole in a foam cored boat?

Just a thought, would setting heli-coils into thickens epoxy work? Might be a bit trickier to get them to sit straight, but I would imagine stronger than a screw tapped into epoxy. What are your thoughts on this CF crew?
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Old 07-03-2022, 18:11   #14
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Re: How to plug a screw hole in a foam cored boat?

I don't think heli-coils are necessary for this. OP is trying to attach window shades with button snaps.
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Old 08-03-2022, 00:45   #15
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Re: How to plug a screw hole in a foam cored boat?

Thanks for all the tips!

I'm in Sydney, so there is about an equal amount of water above the boat as under the boat right now.

Once this storm clears out in a couple weeks, I'll overdrill the hole, clean it out with a bent nail, gorilla tape the back side, fill it with epoxy that is mixed with some filler, give it a whole day to set, then come back the next day and drill some pilot holes and then set some screws by hand that are backed with a bit of sealant, because I know myself and I will mess up at least one of these so why not have the insurance.

Sounds like a lot of work, but I'm totally on board with fix it once the right way and never fix it again.
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