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Old 10-10-2015, 06:23   #1
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Alternative to the "bent nail" technique for holes in cored decks

We all know about the old "put a bent nail/hex bit in a drill" technique of cleaning out rotted wood from around a hole in a cored deck.

What if the wood is not rotted?

I need to fill in several screw and bolt holes but the plywood core is not soft so this technique does not work. I could just pour in some epoxy and create a plug but I'd rather somehow make the plug width go past the hole edges, for greater mechanical strength

I suppose some sort of router or drill bit inserted at an angle?

Any thoughts, hints or suggestions from past experrience welcome
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Old 10-10-2015, 06:44   #2
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Re: Alternative to the "bent nail" technique for holes in cored decks

Get out your Dremel and a cutter bit. Insert into the offending hole and enlarge the under side. They have several types. I use a #115, but I am doing 1/4 holes.
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Old 10-10-2015, 06:49   #3
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Re: Alternative to the "bent nail" technique for holes in cored decks

Hole saw... If you're going to refinish under/around what you are bedding, you can go from the top... If you have access, you can go from below and clean up the appearance with a bunch of different things...
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Old 10-10-2015, 07:18   #4
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Re: Alternative to the "bent nail" technique for holes in cored decks

We also use a Dremel and a #115 bit.
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Old 10-10-2015, 08:12   #5
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Re: Alternative to the "bent nail" technique for holes in cored decks

I use a hole saw to cut through the top layer of fiberglass, pry it out, then use a forstner bit to clear out the wood.

For foam I drill a 3/8" hole and use a cut-down allen wrench in a drill to clear out the foam.
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Old 10-10-2015, 08:21   #6
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Re: Alternative to the "bent nail" technique for holes in cored decks

Since you have to put the #115 in the chuck with enough shaft length to clear the fiberglass skin it is pretty easy to snap the bit off especially if you heat it up too much by doing a lot of holes back to back. Buy a "kit" from Amazon to get a whole selection of bits for the same price as one at Lowe's. The "fill and drill" will be much stronger if you maintain the outer fiberglass skins than if you just drill a big hole and fill it with epoxy by putting duck tape on the inside. I like to use a Rotozip with the #115 instead of a Dremel even though it is single speed, it is a lot tougher than a Dremel.
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Old 10-10-2015, 09:17   #7
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Re: Alternative to the "bent nail" technique for holes in cored decks

I have used a slot router bit chucked in a drill for rebating core around holes before. Probably too big for screw/bolt holes, though.

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Old 10-10-2015, 12:13   #8
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Re: Alternative to the "bent nail" technique for holes in cored decks

A Dremel with the 199 bit. The 199 bit allows you to clean out various thicknesses of core. If you make a plunge cut with the bit angled so it is cutting almost vertical, you mess up very little of the deck. Very useful with small holes and fittings that don't have much overlap around the fastening. They last a surprisingly long time which is unusual working with fiberglass. Have done every fastener on my deck with only two of the bits.
http://www.amazon.com/Dremel-199-Hig.../dp/B00004UDGP
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Old 10-10-2015, 12:26   #9
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Re: Alternative to the "bent nail" technique for holes in cored decks

I would taper the hole slightly,meaning the fibreglass,you could do this with a larger drill,then fill,this will give the filler more surface to adhere to and eliminate cracking later on,there is no reason to under cut the hole.
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Old 10-10-2015, 13:53   #10
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Re: Alternative to the "bent nail" technique for holes in cored decks

drill oversized holes, a pad top and bottom and resin bond your fasteners as in the West manuals
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Old 10-10-2015, 14:09   #11
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Re: Alternative to the "bent nail" technique for holes in cored decks

Quote:
Originally Posted by Cyrus Safdari View Post
We all know about the old "put a bent nail/hex bit in a drill" technique of cleaning out rotted wood from around a hole in a cored deck.

What if the wood is not rotted?

I need to fill in several screw and bolt holes but the plywood core is not soft so this technique does not work. I could just pour in some epoxy and create a plug but I'd rather somehow make the plug width go past the hole edges, for greater mechanical strength

I suppose some sort of router or drill bit inserted at an angle?

Any thoughts, hints or suggestions from past experrience welcome
My boat is foam sandwich above the waterline. Every hole I've made, I've used a hole saw from the inside but not through the outside skin. You can pop the plug out with a screwdriver. Then I've used a putty knife to back fill the hole with a suitable West Epoxy filler mix. Make it fairly thick so it doesn't droop and trowel it in until it comes out the pilot hole on top. Then hold the filler in place overnight with a piece of gaffer tape. If you're using West or similar you can brush a little resin around the interior of the hole before you mix the filler into the resin.
I've usually used a 20 / 25 mm hole saw and then drilled from above for a 6mm /8mm bolt but you can just fill an unwanted hole the same way with out the bolt hole.
You can mask up before you hole saw to keep the surrounding area free of filler. You then drill through the masking tape. That will make the filled hole a little bit; by the thickness of the tape higher than the surrounding area so you might need to sand it smooth later. When I've put a bolt through I've used a 1,1/4" penny washer inside and that overlaps the plug.
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Old 10-10-2015, 16:45   #12
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Re: Alternative to the "bent nail" technique for holes in cored decks

Quote:
Originally Posted by Cyrus Safdari View Post
We all know about the old "put a bent nail/hex bit in a drill" technique of cleaning out rotted wood from around a hole in a cored deck.

What if the wood is not rotted?

I need to fill in several screw and bolt holes but the plywood core is not soft so this technique does not work. I could just pour in some epoxy and create a plug but I'd rather somehow make the plug width go past the hole edges, for greater mechanical strength

I suppose some sort of router or drill bit inserted at an angle?

Any thoughts, hints or suggestions from past experrience welcome
I use a hole saw or drill a bit oversized. Don't go through the inner skin. Fill the hole entirely with Marine Tex. Add glass if you feel its needed. Drill through the entire plug with your smaller hole.
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Old 10-10-2015, 20:10   #13
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Re: Alternative to the "bent nail" technique for holes in cored decks

If you are using backing plates or fender washers , just drill oversize, seal bottom, fill with epoxy, re drill clearance hole for fastener. The "extra" strength of the bent nail trick is negligible and not worth the time and effort.

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Old 11-10-2015, 01:44   #14
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Re: Alternative to the "bent nail" technique for holes in cored decks

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Originally Posted by ramblinrod View Post
If you are using backing plates or fender washers , just drill oversize, seal bottom, fill with epoxy, re drill clearance hole for fastener. The "extra" strength of the bent nail trick is negligible and not worth the time and effort.

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The "strength" is required to prevent compression of the core material when a bolt securing a fitting is tightened. If the core material is compressed the bolt cannot be tightened correctly and eventually water will penetrate the core and in the case of plywood, will rot it. It requires a reasonably wide column of epoxy filler to resist this compression, as well as the working load. Foam will compress. Plywood will compress. End grain balsa resists compression best but doesn't like getting wet. They all need the epoxy treatment. Personally I use a 20 mm dia column of epoxy (using a hole saw) for a 6.5 mm bolt hole. And of course countersink the top of the hole and use a marine sealant under the top of the bolt / fitting. In order to countersink the hole for better sealing there should be an adequate width of epoxy.
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Old 11-10-2015, 11:27   #15
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Re: Alternative to the "bent nail" technique for holes in cored decks

What I have always done...

1) drill through the deck with the right size bit for the hardware. Say 1/4"
2) from the interior drill through the inner layer of glass and the core with a forstner bit the size of the washer you will be using.
3) duct tape the bottom of the hole.
4) fill the void with thickened epoxy, normally a syringe works best for this.
5) let the epoxy kick
6) use the divot from the epoxy shrinkage as a guide to redrill the mounting hole.

I don't like drilling strait through with the larger bit, since you are then solely dependent on the epoxy bond to the hull. This way you also have the bracing of the top skin to hold the plug in place.
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