advice for plywood repair - underinformed

[dechlorinator]

Greetings all - I have a chamberlain dory/skiff, about 14 ft, made of 1/4 inch plywood. Lovely little boat, with a sprit rig and centerboard. (There are no frames - is this "stitch and glue" construction?)

Anyway, due to some neglect and rot, I need to replace a strip of plywood near the gunwale, about 18-24 in long by about 3-4 inches - or roughly half the width of the plank. I also need to patch some fine cracks that have developed, but do not extend beyond the exterior ply of the woor.

I think I can do the former by cutting out the rotted wood and replacing with a glued patch, followed by fiberglass. What materials are good for repairing cracks in the plywood to retain some semblance of strength? I do plan on sailing the boat locally on the Willamette, but crossing the Columbia Bar is right out.

Any advice, direct or links, will be much appreciated. Cheers and such.

james

[Wild Wassa]

If the cracks in the ply are drummy or you can lift the verneer near the cracks or if you run you fingers over the cracks and you can feel ridges, then the rot is progressing along very nicely. Speaking on behalf of the rot that is.

After putting a circular wire brush in a drill and removing all of the soft timber, similar to how dentists do their worst, flood the cracks with CPES (or a similar product although nothing is similar)(not even Evidure)). Then drizzle thinned epoxy in under the veneer and flatten the surface by weighing it down, without distorting the shape. I use mylar sheet as a release for epoxy. Refill with normal epoxy and you might have to use a syringe to inject the voids. Then fair with a filler added to epoxy. That should take care of the cracks. With each of these products above, stick to the manufacturer's advice.

If after removing the strip near the gunnel, it is over a bulkhead or over a stringer, then you have something to nail or screw the new ply to.

If the new strip butts to a chine, it is all too easy. Minimum scarfing is needed for scarfing the ply. Scarf to a ratio of 6:1 -8:1 which is the ratio of the scarf to the thickness of the ply.

"Followed by fibreglass," if you are not retaping, then, is the fibreglass cloth really necessary? ... a good epoxy join is just that. If you feel that sheathing the new work in glass is appropriate then do so.

Good luck and post shots of your boat and the restoration progress.

CPES is made by Smith and Co and is 'Clear Penetrating Epoxy Sealant'. Read the data sheets well. CPES is a rot preventative, that encapsulates existing rot and the fungal spores. I find CPES works well, no matter what the non users of the product here on the Forum say.

"underinformed" ... there is a whole world of restoration exploration out there. I'd hate to think that there wasn't lots still to learn.

Warren.

[Cuyahoga Chuck]

With plywood that has no rot the ideal joint is a scarf. A scarf is a tapered joint where the face of the scarf is about 8 times the thickness of the plywood. For ¼" that's 2 ".
So if you hack out all the rotten stuff and have enough left to grind/rasp/chop/hack/whatever the 2" wide taper all you have to do is make a mirror image repair piece and epoxy it in.
The sticking point is the curve of the plank. The repair piece may or may not accept the curve of the original piece. You may have to back the repair piece up with some kind of plug to get the curve you are after.
Feehanding all the tapers required around the repair can be rather time consuming but if you get even a fair match between the parts and glue the stuff together with thickened epoxy you shouldn't need any fiberglass reinforcement.
I'm am not a CEPS fan. Cutting the bad stuff out is so much cheaper. And cheap I am.
The cracks in the ply surface sound like "checking". It is a common occurance with plywood made from fir. The only long term solution for checking is to cover the panel with a lightweight layer of fiberglass.

[Bob Smalser]

Go over the edges of the bad areas with an icepick and insure you remove all the wood that's soft and punky. Whether your repair is 24" long or 36" long is moot compared to leaving bad wood in the boat. Rot is catching.

Then repair is simply a matter of sawing, feathering, making a feathered patch to match, gluing and fairing. Prebend in boiling water any long pieces that will need to curve in place. It's better to flatten an overbend with clamps until the glue cures than fixing a flat spot with short-lived fairing compound. Feathering to match is easily done using just a bevel gage and block plane with some trial fitting.

http://pic20.picturetrail.com/VOL12/1104763/8408305/113344752.jpg

http://pic20.picturetrail.com/VOL12/1104763/8408305/113600042.jpg

http://pic20.picturetrail.com/VOL12/1104763/8408305/113600038.jpg

http://pic20.picturetrail.com/VOL12/1104763/8408305/113600037.jpg

http://pic20.picturetrail.com/VOL12/1104763/8408305/113600032.jpg

http://pic20.picturetrail.com/VOL12/1104763/8408305/113600127.jpg

[Songololo]

Bob, how far from the rot should one cut back into the good wood when dealing with plywood?

I vaguely recall reading somewhere that, in solid timber i.e. non plywood, that the rule of thumb is that one should cut back 2'' perpendicular to grain and 2 ft along grain into fresh wood as the rot spores have already migrated beyond where the rot-fresh wood boundary is visible.

[Bob Smalser]

I vaguely recall reading somewhere that, in solid timber i.e. non plywood, that the rule of thumb is that one should cut back 2'' perpendicular to grain and 2 ft along grain into fresh wood as the rot spores have already migrated beyond where the rot-fresh wood boundary is visible.

Two feet sounds way excessive to me. "Rot spores" are everywhere, not just in the fresh wood adjacent to punky timber. The trick is to keep them from blooming with proper construction, drainage and coatings.

http://pic20.picturetrail.com/VOL12/1104763/18169354/285438314.jpg

Rot only spreads when wood is at 20% MC or higher. In a properly cared for trailer boat, 20% is difficult to achieve. Why rot spreads even in the best of conditions is that rotten and punky wood is hygroscopic, and remains wetter than adjacent good wood.

http://pic20.picturetrail.com/VOL12/1104763/18169354/285438316.jpg

Obviously the wood in my first two pics showing extensive Brown Rot has to be completely removed back to sound wood by at least a few inches (but not two feet).

http://pic20.picturetrail.com/VOL12/1104763/18169354/285438141.jpg

Catch it early like the "incipient rot" shown beneath this White Rot mycelium, and only the few millimeters of light-colored punky wood have to be removed, and any remaining dark mold scrubbed off with bleach.

What CPES does is seal the repaired and any suspect areas thoroughly, reducing or eliminating the possibility of those areas achieving the 20% MC required for rot spores to germinate and flourish. Applying CPES to those areas is a good practice, and it doesn't take much.

[Thorne]

Welcome to the Forum, James!

The Chamberlain dory skiff is a great design -- well worth fixing up. And Bob's advice is spot-on, you won't go wrong following it.

I've got one that I row and sail all over Northern California. Here's a photo from 3 days ago on Elkhorn Slough -
http://www.luckhardt.com/elk_mesailingweb.jpg


http://www.luckhardt.com/dory2.html

[dechlorinator]

Wow folks - thanks for the great information! I've already "explored" several of the cracks with the Enemy of All Wood (the dremel tool, cone bit) to see if they penetrate the first ply - the CPES stuff sounds solid. Interesting how easily I can feel the difference between good and rot with a hand saw. Recommendations for filler anyone?

Scarfing I can do (not just donuts), and along the other edge I have clear solid gunnel to attach the other edge of the patch to. I'm including glass for the scarfed edge because I'd like the extra structural insurance.

Boiling and bending wood sounds straightforward following anticipated experiments.

I've removed the paint to bare wood in places that need repair, and taken off a good layer (I can discern at least two) elsewhere - for a repaint, is full paint removal required/recommended/ridiculous?

I have a big box store nearby that sells fiberglass & epoxy repair kits, and I assume that an oil-based exterior paint would be best. I'll get some before pics together, along with later pics.

Thanks again for the great response!!

j

[Cuyahoga Chuck]

Wow folks - thanks for the great information! I've already "explored" several of the cracks with the Enemy of All Wood (the dremel tool, cone bit) to see if they penetrate the first ply - the CPES stuff sounds solid. Interesting how easily I can feel the difference between good and rot with a hand saw. Recommendations for filler anyone?

Scarfing I can do (not just donuts), and along the other edge I have clear solid gunnel to attach the other edge of the patch to. I'm including glass for the scarfed edge because I'd like the extra structural insurance.

Boiling and bending wood sounds straightforward following anticipated experiments.

I've removed the paint to bare wood in places that need repair, and taken off a good layer (I can discern at least two) elsewhere - for a repaint, is full paint removal required/recommended/ridiculous?

I have a big box store nearby that sells fiberglass & epoxy repair kits, and I assume that an oil-based exterior paint would be best. I'll get some before pics together, along with later pics.

Thanks again for the great response!!

j

I live in a major urban area and I have to drive 25 miles to West Marine to get a gallon of epoxy. So, be very careful what you buy at a bigbox store. In those places you have to read the the teeny-weeny print on the back of the can. Terms like "fiberglass resin" don't mean zip. If there is epoxy in there it will say so on the can. And it will be expensive. You don't get epoxy for $19.95 a gallon.

[Bob Smalser]

If you're in Oregon make your acquaintance with Fisheries Supply in Seattle. They do mail order.

[Wild Wassa]

"Scarfing I can do (not just donuts), ..."

Well Skipper, I like doing donuts too, closing head inlet and outlets mostly. Do you have a better technique than this?

Cut the donut scarfs on the outside of the hull, then cut another on the inside of the hull. So that dual strength is built into the repair. Scarf 2/3rds of the way deep, from trhe outside and scarf 1/3rd of the way deep from the inside so that the repair is reinforced from both sides. On the outside cut the scarf 8:1 and on the inside of the repair cut 6:1. The quote that I read and liked was, "incase one side takes a knock, the other backs it up."

The Dremel, my favourite and most used tool of all time, is not the one for this type of repair. Use either a pollyfan on a grinder, or a flapper in a drill to cut the scarfs ... the photo below.

The Polyfan is the disk, the Flapper is the loose leaf barrel sander. Too easy with the right concomitants.


http://i225.photobucket.com/albums/dd301/WildWassa/Accoutrements.jpg


Warren.

[P.L.Lenihan]

Warren,
First time I've ever seen this thing called a Pollyfan.What is(or are) its' advantages over a straight disk and who makes it.Does it come in various weights(grades) ?

Thanks,

Peter

[Wild Wassa]

The photo doesn't show the polyfan well. I'll take another photo. Polyfan is the manufacturer's name, they are also called wafer disks.

Polyfan with rubber cleaning block. I use a rubber disk cleaner on the fan from time to time, but not often.

Photo coming.

The polyfan is slightly conical, the individual plates do not sit flat so that the grinder can be held at an angle off the work, while the plates sit flat on the work. They cut with their trailing edges. They cut very quickly with the lightest of pressure.

Before, during an after using the polyfan. This is the same boat that is on the Oxalic Acid thread.


http://i225.photobucket.com/albums/dd301/WildWassa/AKeel.jpg


Polyfans come in a range of grades. A polyfan will remove paint from metal, like a keel ... with not that much extra effort than blowing sawdust off a table (relatively speaking). I view polyfans on a grinder as the ultimate stripping tool. The way they work, without a sharper/deeper cutting edge that disks give to the strip, allows me to be very accurate with what I strip, something that I didn't find when using a conentional disk.


http://i225.photobucket.com/albums/dd301/WildWassa/AAKeel.jpg



Keel ready for a Phosphoric Acid wash.


http://i225.photobucket.com/albums/dd301/WildWassa/AAAKeel.jpg


They are about 4-5 times the cost of an equivalent grade of conventional disk. Polyfans last a long time.

Warren.