encapsulating punky plywood

[farwesthoops]

I know this post has the potential to unleash a torrent of opinions but I am out of fresh ideas. Here is the situation I am facing....I am rebuilding the cabin, cockpit and 2 frames on a derelict Bingham Allegra. If you aren't familiar with them they are 24' Flickas..cousins to Dana 24's. My transom is 1/2" glass lined with 1/2" ply which was total mush due to long time leaks from under the coaming.. I have removed all of the rotted ply I can by slick/drill extensions/chisels/grinders/wire wheels/halibut gaffs/cursing and more. I have a square foot of wet but not punk ply directly behind the molded glass cockpit liner. I simply cannot reach the remaing ply to remove it and absolutely will not cut thru the transom skin. I am considering building a barrier around that last bit to isolate it and then finish repairing the rest of the ply by usual methods. There is no danger of the punk spreading any further. What to do??? It is almost August and I am very frustrated with this. I have searched threads/products constantly for weeks now. All educated opinions gratefully recieved. Thanks in advance. I think I will go get under cover as I expect a deluge of replies as we forumites are never short of opinions..Thanks again steve

[Bob Smalser]

...There is no danger of the punk spreading any further....

What makes you think that? Polyester resin doesn't breathe at all. That's why when water gets in from inadequate maintenance of hardware bedding and drilling holes improperly, the wood cores quickly disintegrate. Epoxy breathes some, slowly passing water vapor, and there's no controversy about either.

Can you thoroughly dry out the bad section before sealing? Because once the punky plywood gets damp...and I suspect it's damp now where you can't reach...it will remain damp...much more damp than the surrounding good wood...and any moisture contents above 20% coinciding with temperatures above 55 degrees will rot all the wood between any polyester and (probably) epoxy barriers.

You could try drilling test holes for air and to measure moisture content, soaking the bad wood with CPES followed by poured epoxy to isolate it, then sealing the holes, but I suspect you'll be ripping the transom apart to do a thorough repair sooner than you'd like.

But I'm a wood, not a glass guy, and don't know much about foams, injection, encapsulation and such. The professional fiberglass guys reside here, and I'm sure deal with problems like yours routinely:

http://boatdesign.net/forums/

[The Bigfella]

The only way I've ever seen this dealt with is to do what you say you will not do - that is, to cut throught the transom skin. What is the problem with doing it that way? I don't wish to sound too harsh, but it seems that you value appearance over integrity.

[Pericles]

Take a look at the rebuild of a catboat here. http://www.devlinboat.com/constructioncatrehab.htm

You will see that cutting off the rotted transom could be the quickest way for you to go.

Good luck,

Pericles

[SamSam]

What makes you think that? Polyester resin doesn't breathe at all. That's why when water gets in from inadequate maintenance of hardware bedding and drilling holes improperly, the wood cores quickly disintegrate. Epoxy breathes some, slowly passing water vapor, and there's no controversy about either.

I believe it is the other way. Poly breathes and results in gelcoat blisters, a supposed remedy is a waterproof epoxy barrier coat. All wood/resin/glass theses claim the same properties.

[Dave Carnell]

Both epoxy and polyester transmit water. The only plastics that are not water-permeable are the hydrocarbon resins-polyethylene and polypropylene.

[Bob Smalser]

Both epoxy and polyester transmit water. The only plastics that are not water-permeable are the hydrocarbon resins-polyethylene and polypropylene.

I won't argue with that. And I'm a forest biologist, not a chemical engineer.

What I do question is how much and how fast polyester and epoxy transmit water vapor. From my experience, wood is much more likely to rot behind a polyester barrier than an epoxy barrier. I believe that's what makes automotive Bondo such a disaster when used to fillet wood.

http://pic20.picturetrail.com/VOL12/1104763/6771586/104804800.jpg

In our cold climate, you have to try real hard to get acidic White Oak heartwood to rot in small, open boats exposed to airflow. Polyester compounds like Bondo make that possible, even easy. Once the wood gets wet above the 20% EMC at 55 degrees or higher it takes for mold and fungus spores to germinate, its ability to dry behind polyester loses the race to those spores already present in the wood. Epoxy fillets don't have the same problems, and I've repaired too much rot behind Bondo over the decades for this to be an isolated incident.

[Brian Palmer]

Steve,

Please clarify whether:

This is a fiberglass boat with fiberglass skin and cockpit liner.

The remaining punky plywood that you cannot remove is isolated from other wood that could become rotten.

If the above are correct, the biggest concern to me would be the potential for the plywood to become totally saturated and then expand if it became frozen, doing further damage.

-- Brian

[Todd Bradshaw]

As far as I know, there is not, nor has there ever been, a polyester boat resin formulated to stick to and/or seal wood. When they build a fiberglass boat, they use the same laminating resin to bury hunks of plywood in the layup as they do to laminate the glass layers and believe me, adhesion to wood was not why they chose that resin. If anything, it "breathes" (a very inappropriate and misleading term to use when talking about either polyester or epoxy resins) more than epoxy will (actually "leaks more" would probably be more accurate). This is due to it's substantially greater shrinkage during the hardening phase and the tiny holes this leaves in the layup unless they really lay it on thick. You can make a pretty decent water barrier with polyester resin and fiberglass, but glass boat manufacturers seldom bother to build up the inside layers over plywood with enough stuff to do a proper long-term sealing job - which is why junk yards are full of old glass boats with rotten plywood in their transoms. Epoxy will seal better with less thickness and at the same time, it sticks and stays stuck to plywood much better than polyester.

In any case, if you have poorly-sealed plywood in a fiberglass layup and water gets in, you WILL have a problem and it doesn't make any difference whether it's polyester resin, vinylester resin or epoxy resin. They're all going to rot. The epoxy is somewhat less likely to delaminate, but the wood will wind up just as punky and you're dreaming if you think any resin type is going to "breathe" enough to save you.

[farwesthoops]

The boat is a very sturdy fiberglass hull which is why I don't want to go in thru the outer skin.. The plywood in question is a piece that reinforces/strengthens the glass transom. It is completely enclosed by a fiberglass inner skin which I have cut away to expose and remove the bad ply. There is no contact nor any way for there to be any contact with other wood any place.. It is simply a piece of disintigrated plywood inside a fiberglass envelope. Thanks all for your responses. The project is interesting to say the least.

[Thorne]

As Bob S said, this sounds like a question for the 'glass guys forum -- they've got to deal with this sort of thing a lot, whether the core is balsa, ply or other material.

[Cuyahoga Chuck]

Rot is caused by microbes that infiltrate with the water. As long as there is suffiicient water present the microbes live on and do their dirty work. The trick is too thoroughly dry the plywood and encapsulate with epoxy so the microbes die.
But, it may be that what's left is not sturdy enough to support the epoxy coating. If the coating does not remain tight you are headed for trouble again. And if the glue bond of the ply is gone it's not up to doing it's job in the structure.
Plywood glass laminates are still fairly popular ( see NA Jacques Mertens)because plywood is a lightweight yet stiff, stable structure. Glassing it on both sides increases panel strength greatly. An all glass laminate requires a lot of layers to be as stiff and it is much more expensive and much heavier. If you have panels where the plywood core is gone what is left is no longer up to the task.
If you have the panel opened up the remedy is a new piece of ply suitably epoxied in and recovered or a bunch of layers of a heavy glass like 13 oz. biaxial laminated on with epoxy. Either way a lot of work.

[Uncle Duke]

OK, you really don't want to go through the transom and get it all out and replace it, so you're looking for an alternate 'fix', even if it is non-optimal. Fair enough.
There is one thing you might try which is better than just leaving the wet wood there:
IF the wet wood is really covered by fiberglass on all sides and IF you can reach all the opening where the remaining wet wood is.... then you could just drill as many holes into the wet wood as possible and then put a vacuum pump on it. Let it run for just as long as you can stand - days if possible - and suck out as much moisture as possible. Then just fill that sucker with epoxy. That MIGHT mitigate the problem. Just be sure that you're not going to collapse something else in the process.
I've seen that tighten up other bad spots, but I have no idea how long the repair lasted, etc. "Last Resort" kind of stuff - you know? All disclaimers apply.....

[outofthenorm]

Bob said it already, but I'll reinforce: after you've dried it out as best you can, you need a saturating style epoxy (like CPES) first, then an encapsulating layer. Another thought - the original plywood was there to create a strong, inflexible "sandwich" of FG/plywood/FG. Have you thought about how you'll put strength back into the transom panel? An un-backed panel of FG is pretty flexible.

- Norm

[farwesthoops]

Thanks all for your insights. To continue on..I intend to epoxy new ply back to replace what I have removed and then glass over that thus duplicating the original work. However, the sq foot of damp ply that resides and hides behind the cockpit moulding is my problem as I cannot get it out.. My inquiry was how to/if I can isolate it and build around it. Bob Smalser, whose opinions I treat with absolute respect, is right on the money as some of you have also pointed out. I will dry it, will possibly use acetone to flush what surfaces of the old ply remain exposed, and understand the permeability of all resins. Of course the easiest solution by far would be to simply find a way to get that last bit of old ply out but so far I haven't solved that issue..that is the real dilema. Surprisingly, Alan Vaitses in his book entitled The Fiberglass Boat Repair Manual, mentions a couple of projets similar to mine where they could not get all the wet ply out. They sealed it with hydraulic cement and rebuilt what they could get at! I know we are a Wooden Boatforum and I am viewing this whole thing as replacing wood rather than repairing a glas boat. Thanks again to all for your help. On we go...

[outofthenorm]

One last thought about using the saturating type epoxy. The following safety consideration was pointed out to me privately, and I think it's a good point:

"Don't use a solvent based epoxy is a confined space where the solvents cannot flash off and evaporate! The worst case scenario is an explosion!

Instead use a thin solvent free epoxy that will bond to wet or damp surfaces."

To me it means choose your epoxy with care, and that if you use CPES, make sure you have serious ventilation of all low-down spaces. Even a small explosion can ruin your whole day.

- Norm