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Old 01-18-2015, 05:38 PM   #1
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Sealing plywood cored thru holes

I have to drill several holes through my foredeck to mount the anchor windlass pedestal, the deck being fiberglass over plywood over fiberglass, perhaps an inch and a half thick. I need to seal the wood core, but have no idea how to go about it in the best way. If it were a balsa core, it's well documented. Can I just use something like "Git Rot" as a penetrant sealer for my 3/8" through holes for the base mount?

Drilling oversize holes doesn't seem like the best answer.

Looking for some tips here :-) I will post some pictures of the finished project when done, the project just being at the "fit the pedestal" before final welding on the chain pipe. And boring holes in my foredeck with a drill and a few hole saws...
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Old 01-18-2015, 05:50 PM   #2
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Two or three coats of West (or similar) epoxy? I've done it and seems to soak in to the ply and hold up okay. Take the blush off the epoxy between coats. As to the advisability of thinning it to make in penetrate more see: WEST SYSTEM - Projects - Thinning WEST SYSTEM epoxy
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Old 01-18-2015, 05:55 PM   #3
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Drill oversize and use 5 or 15 minute epoxy to line the hole. Then seal and bed properly using something like 4200.

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Old 01-18-2015, 05:56 PM   #4
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Then seal and bed properly using something like 4200.

Ken
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Old 01-18-2015, 05:59 PM   #5
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I drill oversize by 1/2 an inch or so....tape the bottom...fill with epoxy and filler blended...then I drill though the epoxy only to mount.

It takes longer but it's the ticket...
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Old 01-18-2015, 06:02 PM   #6
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In balsa cores they advise thinning the resin to penetrate the core before filling the void. I was thinking there must be something that will penetrate "new" plywood (not old holes) deeply enough to seal it. The plywood is relatively uncompressable, so the usual routine with oversize holes in a core seems like overkill. Drilling "straight" through the center of the pour seems unlikely as well, unless the hole was hugely oversize.

Brain at work here, thinking about what makes sense.
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Old 01-18-2015, 06:11 PM   #7
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In balsa cores they advise thinning the resin to penetrate the core before filling the void. I was thinking there must be something that will penetrate "new" plywood (not old holes) deeply enough to seal it. The plywood is relatively uncompressable, so the usual routine with oversize holes in a core seems like overkill. Drilling "straight" through the center of the pour seems unlikely as well, unless the hole was hugely oversize.

Brain at work here, thinking about what makes sense.
Thinning epoxy can be problematic unless you are good at it.....and even then there is a debate whether it really does any good.

If you can't mark and drill new holes in something that has a 1/4 inch clearance all around..maybe you shouldn't be doing your own maintenance.
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Old 01-18-2015, 06:19 PM   #8
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OK, another search found the suggestion of using a Dremel to remove the plywood under the fiberglass, like the balsa pick on a drill chews out the fiber. I now have a plan. Thank you all very much for your suggestions.
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Old 01-18-2015, 06:26 PM   #9
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Thinning was fiberglass resin vs epoxy. I had the recommendation from a local fisherman that I use what my boat was constructed of rather than epoxy as much as possible. I was talking WEST to seal and fill my cored roof for a skylight/hatch install in the pilothouse, he pointed out the advantages of matching construction materials. It made sense.

1/4" all around the hole through a non compressable material seems excessive, the sealing is against moisture intrusion, not against compression of the core. The load is in shear, so moisture intrusion is the issue in sealing.
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Old 01-18-2015, 06:59 PM   #10
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Thinning was fiberglass resin vs epoxy. I had the recommendation from a local fisherman that I use what my boat was constructed of rather than epoxy as much as possible. I was talking WEST to seal and fill my cored roof for a skylight/hatch install in the pilothouse, he pointed out the advantages of matching construction materials. It made sense.

1/4" all around the hole through a non compressable material seems excessive, the sealing is against moisture intrusion, not against compression of the core. The load is in shear, so moisture intrusion is the issue in sealing.
You answered your own concern...the 1/4 inch is for no other reason than making sure you can drill through that new area without missing and nicking the core...not for thickness of waterproofing. The compression resistance provided by epoxy and filler is a good thing though.

Hey do whatever....just telling you what works in the yards, marine businesses I work in.

There are faster methods using polyester and or 4200/5200....but I like a more sure bet.
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Old 01-18-2015, 08:38 PM   #11
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Pour in some CPES first to penetrate the wood.
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Old 01-18-2015, 08:44 PM   #12
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You ask how to do it correctly. PSNeeld told you how to do it using accepted best practice. Then you got advice from a local fisherman to do it his way using vastly inferior products and slip shod methods. I dont get it, why ask ?
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Old 01-18-2015, 09:18 PM   #13
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A single reputable author listed 6 different ways to do the job. Some removed more material and some removed less. Some bored a single larger hole through the deck and some a larger hole from the top and a little one from the bottom, or the whole top and bottom as with a hole saw. Replacing the whole core or enlarging the hole too much didn't make sense.

I am using this forum to get my own thought together about the process I want to use to get my job done. I am not being contentious, and professional qualifications have to be noted to be considered. I appreciate all comments and suggestions, they have helped me to form my plan.

Don't question the qualifications of my "local fisherman", he does outstanding fiberglass work well recognized in the local community. His demand far exceeds his ability to meet it.

I am not familiar with CPES, didn't see it in my West Marine catalog. What is it?

My plan is to route out the plywood below the fiberglass with a dremel type bit and fill with thickened epoxy, without enlarging the existing holes in the fiberglass, and then to drill, as suggested.

Sometimes it takes a while to decide what you want to do, and how. Thanks all.
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Old 01-18-2015, 09:45 PM   #14
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I usually drill thru the top layer of FG with a hole saw then pry out the FG plug. Then drill out the plywood with a forstner bit, being carefull not to drill thru the bottom FG, then mop the hole with regular epoxy, then fill the hole with epoxy thickend with cabosil or milled fibers (or saw dust) to a peanut butter consistancy, then press back in the FG top plug. The new hole to be drilled in the holesaw pilot hole. Easy and strong. Much less compressible than plywood. It is of utmost importance to seal wood cores in a similar manner. Wood is always compressable and once it is compressed it tends not to come back completely. If the bolts are tightend on a cold day the wood will be compressed when the deck heats up in summer and expands. This will leave a small gap the next winter that WILL get wet. Rot is inevitable. The epoxy plug is harder and even if compressed will come back, not leaving a gap. Even if water does get under the fitting the epoxy seals it from the wood. Polyester resin will crack if used in this manner. Some poly tooling gells are used for this purpose but rarely.
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Old 01-19-2015, 12:55 AM   #15
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CPES is similar to Git Rot. It is a penetrating Epoxy Resin.


Just today I bolted an outboard to a plywood transom with fiberglass on both sides.
I drilled the hole, pumped it full of 5200 (Girly Men use 4200 ), put a bead of 5200 under the bolt head and fender washer, shoved it through, tightened the nut and washer down into the 5200 that was pushed out the other side, cleaned up the mess with WD40 and a rag. I'm confident that water will not get into the plywood.
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Old 01-19-2015, 01:10 AM   #16
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CPES is similar to Git Rot. It is a penetrating Epoxy Resin.


Just today I bolted an outboard to a plywood transom with fiberglass on both sides.
I drilled the hole, pumped it full of 5200 (Girly Men use 4200 ), put a bead of 5200 under the bolt head and fender washer, shoved it through, tightened the nut and washer down into the 5200 that was pushed out the other side, cleaned up the mess with WD40 and a rag. I'm confident that water will not get into the plywood.

That's how I've always done things. Gobs of 5200 and multiple rolls of paper towel, and GoofOff. Works great for cleaning up 5200 and others sealants/adhesives.
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Old 01-19-2015, 11:10 AM   #17
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I drill oversize by 1/2 an inch or so....tape the bottom...fill with epoxy and filler blended...then I drill though the epoxy only to mount.

It takes longer but it's the ticket...
This is the best way. There are other ways to do it, none better than this. If you don't have the time to do it right, where will you find the time to fix it later.

Ted
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Old 01-19-2015, 11:48 AM   #18
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In all fairness, kulas44 in post 14 probably described the best way I would do it...especially when the fastener or hole size becomes larger. Keeping the lower skin can help a lot...but it does involve a bit more work and tools.

Definitely the way to go if the core is more easily compressible than ply.
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Old 01-20-2015, 10:24 AM   #19
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Another way to solve the problem is to do as suggested , drill oversize and plug with reinforced epoxy then redrill.

Then grind the area where the windlass or stanchion , whatever is to be mounted and epoxy down a flat (store bought) hunk of laid up glass.

At about 1/2 thick it will help spread the load , and ease the loads on the backing plates.

BEST!!! of all it will raise all the seal area under the item by that 1/2 inch and give a really flat area to mount to..

Standing water on deck will need to be over 1/2 thick to even test the quality of the seal goop.
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Old 01-24-2015, 03:39 PM   #20
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I really like the suggestion in Post #14 too, it will be my back up plan if I don't feel like my Dremel bit is getting enough material out for a good seal. My foredeck has a little bit of a hump, so it isn't flat enough for a base plate. I am using washers under the corners of my pedestal base to level it and leave a good layer of 4200 under the base to avoid squeezing too much back out bolting it down.

On another note, the holes through the deck for the chain pipe and for the cable routing also have to be sealed. Since they aren't sealed (open from the bottom) like a bolt would be, is just sealing the exposed edge of the wood with something like Verathane (outdoor formula) appropriate? Moisture could drain without being trapped between layers...

Appreciate opinions :-)
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