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Old 27-10-2015, 20:42   #1
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Having difficulty epoxy potting

I've read lots of guides on epoxy potting and rebedding deck hardware (Don Casey, Maine Sail / CompassMarine) and I love the idea in theory - but find it's not as simple as it sounds in practice. Maybe I'm doing something wrong?

Two issues:

- Epoxy potting in marine plywood core (in high load hardware areas) is way harder than Maine Sail made it sound. He suggests Dremeling out the coring with a Dremel #654 bit (1/4" router bit). I tried that - on the 5/16" holes for a U-Bolt - but it barely put a dent in the plywood. The wood is so hard that I wasn't able to remove much of it.

I don't want to overdrill the holes because it's better to keep the fiberglass skin. Also overdrilling a 5/16" hole (with say a 3/8" bit) can be a bit tricky - a big 3/8" or 1/2" bit is likely to bind on the existing fiberglass skin.

Maine Sail's site says 10-15 seconds of Dremeling was enough to remove plywood core. But I did 4 or 5 rounds of 10-15 seconds and only had about 1/64" removed. In some places the plywood looked kind of burnished / polished (shiny + smooth). I ran the Dremel at 1500-2000 rpm. I tried higher rpm (2500) for a second but that was worse (chattering and hard to control).

I don't think the bent nail / broken allen key trick would work any better.

This epoxy potting attempt was somewhat disappointing because I felt like it perhaps did more harm than good. Epoxy potting without removing enough core to prevent your redrill holes from exposing core is kind of pointless. I wondered if I would've been better off just rebedding the hardware and reusing the existing holes.

- Lining up the redrill hole with where it was before. How do people get perfectly aligned thrubolt holes for U-Bolts? I got the angle close to correct, but the holes were still off about 1/8" on the bottom (cabin) side. This made it hard to get the U-bolt in (and re-exposes core by going outside the epoxy pot).
The deck here is 1/4" FRP, about 3/4" plywood, then 1/8" FRP, then 1/4" foam filler (above cabin liner), then 1/8" FRP (cabin liner). So a couple degrees off on the drill can make a big difference.
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Old 27-10-2015, 21:10   #2
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Re: Having difficulty epoxy potting

With a ply core the potting is not for structure but water proofing. Perhaps you could live with drilling an oversize hole through the top skin so you could have an 1/8" of filler around the bolt hole. Then of course a washer would cover up the filled area.
I met a guy once who said he drilled angled oil holes in the bearing housing for a nuclear reactor generator freehand. He said he hit the right spot from about 8" away through steel with a hand drill. If he could do that you should be able to drill a hole through your cabin roof.
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Old 27-10-2015, 23:10   #3
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Re: Having difficulty epoxy potting

I experienced all the same issues. It required lots of patients and approximately one dremel bit per every two holes to route out the plywood. I assumed that Maine Sail was talking about balsa cored decks when he mention the 10-15 seconds. Which in my experience is true with balsa.

i just persevered.

I also had difficulty using an over sized bit to redrill the holes. I redrilled the holes in reverse gear, not just through the fiberglass skin, but all the way through the ply, otherwise it would bind.

I also used a drill guide from harbor freight. It was a mini drill press that attaches to your drill. ONly cost about $20. Worked great for lining up the whole and redrilling straight through the potted epoxy.
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Old 27-10-2015, 23:28   #4
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Re: Having difficulty epoxy potting

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Originally Posted by Guy View Post
With a ply core the potting is not for structure but water proofing. Perhaps you could live with drilling an oversize hole through the top skin so you could have an 1/8" of filler around the bolt hole. Then of course a washer would cover up the filled area.
I met a guy once who said he drilled angled oil holes in the bearing housing for a nuclear reactor generator freehand. He said he hit the right spot from about 8" away through steel with a hand drill. If he could do that you should be able to drill a hole through your cabin roof.
With overdrilling the fiberglass I just feel like I'm doing more destructive work with potential to mess something up, to fix a problem which may not even exist. There's some truth to the saying "if it ain't broke, don't fix it."
And any additional steps I do like overdrilling have potential for something unexpected going wrong - that nuclear reactor guy was way more skilled and experienced than I.

But if there were a simpler way to ream out plywood core (like a different Dremel bit) or if I were doing something wrong, it'd still be worth doing. Otherwise I think it may be worth only potting balsa cored deck hardware, and just properly bedding plywood hardware and occasionally checking their underside for water or rust. Plywood core doesn't rot overnight, it takes months (or years?) of neglect.
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Old 27-10-2015, 23:44   #5
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Re: Having difficulty epoxy potting

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Originally Posted by Tessellate View Post
With overdrilling the fiberglass I just feel like I'm doing more destructive work with potential to mess something up, to fix a problem which may not even exist. There's some truth to the saying "if it ain't broke, don't fix it."
And any additional steps I do like overdrilling have potential for something unexpected going wrong - that nuclear reactor guy was way more skilled and experienced than I.

But if there were a simpler way to ream out plywood core (like a different Dremel bit) or if I were doing something wrong, it'd still be worth doing. Otherwise I think it may be worth only potting balsa cored deck hardware, and just properly bedding plywood hardware and occasionally checking their underside for water or rust. Plywood core doesn't rot overnight, it takes months (or years?) of neglect.

This. Just seal the ply with penetrating epoxy and call it good. Unless its a large hole you can get a laminate trimmer into, and both skins are thick.
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Old 28-10-2015, 00:37   #6
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Re: Having difficulty epoxy potting

If your Dremel bit is not cutting well and leaving the plywood looking burnished, I would guess the bit is dull.
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Old 28-10-2015, 04:38   #7
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Re: Having difficulty epoxy potting

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Plywood core doesn't rot overnight, it takes months (or years?) of neglect.
Wrong approach.

Just over drill it enough to give you a margin of error fo re drilling the u bolts.

Use good fender washers and/or that big washer that comes with the u bolts, spanning both bolt ends.

As was said earlier, in your case, plywood is the strength, glass is a protective sheath. You could remove all the glass.. the entire glass skin.. and barely change the characteristics of your panel.

So, don't think of this as any different than a piece of plywood you are attaching hardware to. You arr just creating a water tight attachment point.

Over drill, use washers/backing plate and make a repair that will last a lifetime.
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Old 28-10-2015, 04:38   #8
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Re: Having difficulty epoxy potting

maybe one of these cutters instead?

199 HS cutter




655 keyhole


Though I'm with Minaret, if the core is hard enough to dull a cutter it doesn't need removing. Good rebedding (butyl tape?) and secure fastening with sturdy backing plates is probably the best all-round approach.

Regarding drilling holes, get a piece of 2 x 4, draw circles in the same place on the top and bottom surfaces, and practice...
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Old 28-10-2015, 08:44   #9
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Re: Having difficulty epoxy potting

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Originally Posted by minaret View Post
This. Just seal the ply with penetrating epoxy and call it good. Unless its a large hole you can get a laminate trimmer into, and both skins are thick.
I think that's what I should have done. The thought occurred to me but I dismissed it because I was thinking the epoxy drippings would constrict the hole (the 5/16" holes had very little clearance for 5/16" u-bolts, I had to yank them out and pound them in). But I suppose I could've done the partial core dremeling like I did and then epoxy coated, and if any overdrip blocked the hole then I could just drill or sand that. Much easier than fully epoxy potting and redrilling. A q-tip could help get the epoxy in or I suppose I could just pour it in and then drain it out.

My impression from reading about epoxy potting was that part of the purpose is actually to strengthen the deck in balsa areas. That a major source of leaks in balsa decks is when the balsa gets compressed (by overtightening, or just flexing of fittings) and cracks or draws away from the fiberglass.

That won't happen in the plywood reinforced section of my deck - it's pretty bomb proof - at least 1" thick and I don't even use it for any really high loads.

Realizing this I think epoxy potting makes a lot of sense for balsa core, not as much sense for plywood core where there isn't evidence of water intrusion and/or low risk of it.
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Old 28-10-2015, 08:45   #10
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Re: Having difficulty epoxy potting

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Originally Posted by Steve Bean View Post
If your Dremel bit is not cutting well and leaving the plywood looking burnished, I would guess the bit is dull.
It's a brand new bit ordered from Amazon a couple weeks ago that I had never used before.
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Old 28-10-2015, 09:11   #11
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Re: Having difficulty epoxy potting

Try one of the cutters in a cordless drill. Lower RPM. I've done it this way.

For foam, I like the bent nail better. The trick is that the bend must be very sharp, the bent leg very short (1/8-3/16"), and the cut end sharp (not nippers).
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Old 28-10-2015, 09:15   #12
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Re: Having difficulty epoxy potting

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Originally Posted by Tessellate View Post
I think that's what I should have done. The thought occurred to me but I dismissed it because I was thinking the epoxy drippings would constrict the hole (the 5/16" holes had very little clearance for 5/16" u-bolts, I had to yank them out and pound them in). But I suppose I could've done the partial core dremeling like I did and then epoxy coated, and if any overdrip blocked the hole then I could just drill or sand that. Much easier than fully epoxy potting and redrilling. A q-tip could help get the epoxy in or I suppose I could just pour it in and then drain it out.
I've always just filled the hole completely (after reaming) and redrilled. It's quick, easy and overall I spend less time doing the job than fretting over it.

That said, my SOP is drill, ream, add unthickened epoxy to wet the surfaces, fill with thickened epoxy, wait, drill and rebed. So far no problems. And yes, reinforced areas are a bit of a pain to ream, but oh well ....
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Old 28-10-2015, 11:22   #13
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Re: Having difficulty epoxy potting

I'm curious as to what the Harbor Freight tool was (small drill press)? Sounds pretty handy to me, but I did not see anything similar to that description on the HF website.

I second the motion on using penetrating epoxies. I am new to working with epoxies and the two that have been most useful to me working on old boats (of all types) is the penetrating epoxy (I use TotalBoat) and various fillers that get mixed with epoxy to provide enough thickness to use on ceilings without dripping (like micro balloons). I saved the work of replacing motor transoms and decking with rotten cores using penetrating epoxy. Great stuff. And for filling good size holes and rips, fillers like micro balloons and milled fibers do an excellent job of filling the gap and adding strength.

One thing about penetrating epoxies. They pour pretty thin. So you better have something underneath to catch the drippings. Use tape to dam up any holes ahead of time or you will have a puddle of epoxy (the size of a large dinner plate) underneath your repair. Something I didn't catch the first time until the epoxy had cured. I ended up leaving it. Added strength, right?


Also, if you have problems with large drill bits catching onto the fiberglass in thin areas, try using a countersink. Slower, but produces a good beveled hole and does not catch onto the thin fiberglass.
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Old 28-10-2015, 11:29   #14
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Re: Having difficulty epoxy potting

For plywood deck cores, I would just drill the hole 1 bit size over and then tape the bottom, fill with epoxy and let sit for a few minutes to soak in. then remove the tape from the bottom and drain out the excess. Let harden and then just "size" it with the desired bit.
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Old 28-10-2015, 12:39   #15
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Re: Having difficulty epoxy potting

The Dremel 199 bit works a treat. If you make the initial penetration at as close to 90 degrees as you can and then vertical after you are through the FRP deck, you make a minimum mess of the surface and the fitting will probably cover your work. The 199 bit has worked to remove plywood core just fine for me.

Your really want to remove balsa or other compressible core. If you tighten the bolts too tight or the fitting is subject to high torque loads, a leak is often the result. Beveling the hole edge and using a good quality caulk will go a long way to ensuring there is no leak. Potting' the core is the only sure way that no water will get into the core, however.

As far as taping the underside before pouring in the epoxy, use a good sticky tape like duct tape. Masking tape will hot stay in position and it's a real PITA to clean the epoxy off your teak and holly cabin sole, btdt.
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