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  #1  
Old 03-10-2004, 09:23 PM
willibuch3 willibuch3 is offline
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plywood, to seal or not

Has there been a consensus formed on the sealing of plywood hull planking (new construction)? More specifically; if a hull is epoxy and cloth coated on the outside are there good arguements to coat the inside as well to prevent water absorption? Or is it better to leave the inside bare? I am going to be facing this decision in a few months. I lean toward coating the inside to avoid water soaking into all the nooks and crannies always found in boat hulls which are good places for rot to start. There are two popular books which have opposite opinions on the subject.

Anyone have experience either way>

Bill buchanan
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  #2  
Old 03-11-2004, 12:24 AM
captword captword is offline
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go look at some older boats in the repair yards. If they will let you look into the inards, do it and see for yourself. Ive done repairs on old bertams, hatterases, and other glass vesels some old enough where they didnt completeley cover all the plywood in what they though were noncritical areas. you can see the diference in the quality between the parts that were partially covered and those that are not. something that i may add that i like to do on inboards is to epoxy in reg matting under the engine and other areas where I know future work is going to be done. This keeps a dropped wrench or hammer from cracking your seal to the ply.
howard
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  #3  
Old 03-11-2004, 11:33 AM
willibuch3 willibuch3 is offline
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plywood, to seal or not

Howard,

Thanks for your insight. The spot application of glass in more susceptable areas is a good idea. I tend to think "all or none" when considering boat coverings and coatings.

Bill Buchanan
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Old 03-11-2004, 11:56 AM
Kyle Kyle is offline
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I can't say if you should or shouldn't coat, but I wanted to mention to look at the edges of the plywood. The sides are like open capilaries ready to soak up moisture and water vapor. This action creates rot from the inside out. CPES is said to help
considerably with this problem.

Kyle
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  #5  
Old 03-11-2004, 04:09 PM
lprimina lprimina is offline
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I like to over kill, so seal
Ben
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  #6  
Old 03-12-2004, 07:00 AM
GordMay GordMay is offline
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Encapsulating Plywood

I too, would be uncomfortable leaving plywood un-sealed - but have often wondered about "out-gassing" of the interply bonding agents.
Is epoxy permeable to urea formaldahyde (or whatever they're using these days)?
If not, might we not worry about the internal pressures that could build up inside the encapsulated assembly - or am I just "blowing smoke"?
Gord
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  #7  
Old 03-12-2004, 07:32 AM
captword captword is offline
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the epoxy will stick to it. what happpens in exposed wood is that it soaks up moisture out of the air and expands and contracts according to its moisture state. Over time this expansion and contractions will weaken where it is bonded and given enough time weaken its bonds within itself. when you completly encapsulate it you lock it at its curent size. It cannot get bigger because it cannot get any extra moisture in it for expansion, it cannot get any smaller because it is sealed in the same way. as far as off gassing from the ply glue. unless you live right next to the factory and get it hot off the press, the off gassing is far and well beyond the point of being any problem.
howard
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  #8  
Old 03-13-2004, 12:40 AM
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When epoxy is first put on ply or lumber it will maintain the moisture content at the state it found the material. That is the advertising, it locks down the stability of the material.

In the real world there isn't any such thing as total encapsulation. A corner bashed into a frame as the piece was installed, the dropped wench, life in general WILL breach the coating, then the moisture will seek out the innards of the lumber or ply and the stuff HAS to find a balance. During this balancing process as the material stabilizes on it's new moisture content things move. Just ask anyone who epoxied Doug fir ply thinking it would stop the checking. You have to stop the movement or deal with the movement. A layer of cloth set in a matrix of epoxy will keep Doug fir from checking.

Epoxy isn't a cure all, in fact more problems can be created by using the stuff then prevented. Coated ply WILL be breached if the boat gets any use at all, then the coating acts against you by trapping the moisture in the material. We all know what this leads too.

Adding a bit of cloth or mat for abrasion resistance in high traffic or ware areas is a good idea. Encapsulation is a good idea if the design is intended for a homogenous structure, but if the structure was not intended as a solid , but in fact as a collection of pieces working (read moving) together to be a boat, then use the stuff for what it's good for and not try to make a silk purse from a sow's ear.

Epoxy has some wonderful qualities and I use it all the time, but I don't ask of it things it wasn't designed to do. It's sticky stuff, but there are stickier and CPES is a good primmer, but there are better. In some uses, it's hard to beat, but I've found most over use it and don't understand the real properties of the material.

The epoxy I used today is a 5:1 and has these properties;
Barcol hardness 38 (Shore D 80)
Tensile strength 14,600 psi
Tensile modulus 497,000 psi
Tensile elongation 3.4%
Flexural strength 17.700 psi
Flexural modulus 697,000 psi
Compressive strength 14,600 psi
Water absorption (21 days) .05%
(bet you thought it was water proof didn't you . . .)

In short to answer the origional question, it depends on the application and design requirements. A taped seam boat design wouldn't be for not the invention of epoxy, but some other common uses may not be well advised.
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Old 03-13-2004, 01:47 AM
betelgeuserdude betelgeuserdude is offline
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Bill, with respect to your original inquiry, I'm of the school of thought that one either attempts to seal everything, or let everything breathe, aboard a stable plywood boat. I believe that IF you epoxy (with/without fabric) the exterior of a hull, you must also apply epoxy to the interior. There are many examples of plywood boats built several decades ago which used no epoxy or any other sealer. Sure, some used pentachlorophenol, some used copper naphthenate, but the wood was unsealed.

One only has to remember the loss of many old, canvas covered, cedar canoes as well-intentioned folks stripped off the canvas and "fixed" the canoes with glass and resin. Not only did it immediately destroy the inherent value of the boats, it hastened their deaths.

As Par rightly points out, the wonderful benefits of epoxy can also be detrimental to the material to which it is applied, same as LPU paint jobs. A little bruise to unsealed (or enameled, varnished, oiled, etc) wood will have no ill consequence, while a bruise to epoxied or LPUed wood may microscopically damage the coating to the extent that water intrusion occurs. It may not even be apparent until years later when someone steps through the still beautifully finished, but completely rotten structure. Still, everyone faults the construction material rather than the finishing material when this happens.

I generally like to apply epoxy and fabric to my own decked plywood boats, while I choose to merely enamel skiffs and the like. Open boats are more likely to get damaged, so I rather them be frequently repainted, and easily repaired when necessary, rather than epoxied and forgotten.

Regardless what you end up doing, definitely spend some serious time considering ventilation. A boat must breathe, or it becomes a mushroom farm.

DC
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Old 03-13-2004, 08:08 AM
Kyle Kyle is offline
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I have to agree 100% with Par. I know that I have been the CPES poster child lately. Water that finds its way into ply or wood thru a micro-crack creates a wonderfull hotel for rot.
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Old 03-13-2004, 04:29 PM
tonyr tonyr is offline
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I am facing this decision in a strip built 24 ft power boat. The received wisdom with this method is that you should thoroughly fibreglass and encapsulate the outside of the hull, and consider the same for at least the bilges inside. The difference between plywood and glued/strip is that if (when!) the seal is broken inside with glued strip, the moisture which gets in cannot migrate past the nearest glue line (say max 3/4" away). Thus with glued strip, encapsulating is safer than with plywood, since rot is localised.

Regards, Tony.
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Old 03-13-2004, 07:57 PM
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As a very general rule I cloth and epoxy the outside of trailer boats. The idea is to get the best of the method and materials, the abrasion resistance of the cloth in epoxy and the water resistance of the epoxy. I'll leave the inside of the boat painted or with some areas clothed up, again only for the added abrasion resistance. This assumes the boat will get wet, dry, live on a trailer and be covered.

On light weight boats, car toppers, skiffs, small day sailers and the like, I opt for a good painted finish. The extra effort and expense of the goo, doesn't add up in my book, not when you are so close to the structure that every inch can be hand dried in a capsize or big rain when the cover got left off when the weather man lied about the following morning's forecast.

On moored/docked craft I tend to be a bit conservative as the paints that have served so well haven't bitten me in the butt as much as the "hard" coatings that have cropped up in recent years, including epoxy and CPES (which I've all but stopped using, unless requested) Traditional construction CAN'T have these coatings, period. The movement is to great, even for the almighty epoxy. The structure requires the parts to work, it's designed into the structure and trying to lock them down is foolish.

More modern construction methods, require and some wouldn't exist without epoxy. There, cloth/mat in an epoxy matrix must be used as the structure requires it to be a solid, not a bunch of working parts. If the parts do move the boat is in danger, so they must be locked down. In these boat designs the thing should be dipped into a vat of goo and sheathed, per the designer's intent.

On a plywood boat NOT relying on tape and seam techniques for it's structure, I'd use cloth set in epoxy on the outside for the abrasion resistance and in high ware areas inside, but I'd not get carried away with the stuff. This will allow the structure to breath and adopt the natural moisture content balance it must, without busting out a glue line or joint in the process. Tape and seam, just soak and sheath, you're building a milk bottle anyway.

TonyR, I'd be careful about skinning both sides and thinking rot will be localized. I've never seen rot, if left to it's own devises stay localized. The fasteners holding those strips down as the glue sets (edge set nails) will provide a great way for rot to get to the next layer. In most current thinking strip built designs, the idea is to use the cloth/epoxy as a structural skin, basically a core of strips with an inner and outer skin of glass. This is the same concept as foam core or balsa core construction, though strip cores are much stronger then some of the other cores used, it also weighs more. Stick to the designer's layup schedule and you'll be okay. Check with them (the designer) if the design is older the 10 years, and see if their thinking is still the same on the layup schedule for the glassing.

One last point is, epoxy is near worthless without the fabric matrix unless used as an adhesive.Alone it will protect little except as a barrier coat for glass hulls that need it. Some use it under varnish, I don't, though my master bath counter top does look great in the stuff, it will not be asked to carry me farther from shore then I can swim, nor will it receive direct sunlight.
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