The right plywood for interior work

suntreader · Post by **suntreader** » Fri Oct 26, 2007 4:39 pm

I haven't posted in a while, but my Islander 32 project is moving right along. We've been grinding/sanding for months now and we're about done with the interior. The next step after a major wash is going to be a lot of epoxy and tabbing work where some bulkheads have separated or gaps or what not. Once that is done I will start rebuilding the head, dinette, quarter berth, and galley. The original plywood was mostly 3/8 inch with a mahogany veneer. What plywood is suitable for the interior rebuild? Regular exterior plywood would need veneer to look decent and I don't want to take that extra step if I don't have to. The local Lowe's and HD have cabinet plywood that I have used plenty of for furniture; birch, oak, and 'sandply.' This stuff would work fine except it only comes in 1/4", 1/2", and 3/4" and it seems like a bad idea to use interior ply on a boat, even on the interior. Real marine ply would seem like overkill.

On the Glissando site Tim used cherry ply, which if I'm not mistaken is also made with interior glues. If that has held up then I may go with some 1/2 and 3/4 birch, that would be the easiest route.

Anyways, interior plywood, what does everyone use?

Thanks,
David

jollyboat · Post by **jollyboat** » Fri Oct 26, 2007 5:14 pm

It is my understanding that there is no difference in the glue used in the construction process of plywood. Marine plywood is constructed with laminates that contain no voids. The reason for this is so that the plywood does not fail in areas of the boats construction where there are both twisting and bending angles - voids would either buckle or collaps in these areas if stressed enough (pounding from waves). Many small boats like skiffs and dorys (Dynamite Paysons - Light Dory for example) can be constructed from standard A/B or B/C exterior grade plywood. Furnature grade or "interior plywood" is a fine material for boat interiors. It is typically constructed with a greater amount of laminates to provide enough strength that is needed in either semi-monocoque or monocogue construction that is often found in cabinatry. For bulkheads that will be tabbed in with glass - exerior grade plywood would be just fine. After it is installed, how it is finished is up to you. I think that beadboard is very traditional and very easy to install in that it comes in 3.5" widths so the fitting in is very easy. For non-load bearing areas in the boat, such as cabinatry - luan would be just fine if you are inclined.

Below is some info that I poached from the capital city website.
http://www.capitolcitylumber.com/plywood.cfm

Construction Grade Plywood:

CDX: Lowest grade of exterior plywood available usually used for sheathing on roofs or walls. Can be used for utility purpose. Available in 3/8", ?", 5/8" and ?"thicknesses in 4x8 pieces.

5/8" Underlayment: 4x8 used for subflooring, when a smooth surface is needed. This plywood is made with exterior glue.

BC Plywoods: BC grade plywoods have one finished face usually yellow pine made with exterior glue and is available in 3/8", ?", and ?" in 4x8 pieces.

Finished and Special Purpose Panels:

AC Plywoods: AC grade plywoods have one finished face usually Douglas Fir made with exterior glue and is available in ?", 3/8", ?, 5/8" and ?" pieces.

Luan Plywood: A cheap grade of Mahogany plywood available in 1/8" and ?" 4x8 pieces. Perfect for Cabinet backing, or any utility purpose.

A/B Marine: Marine plywood available in ?", 3/8", ?" and ?" thicknesses 4x8 sheets (sold by the sheet only). Marine plywood is usually used in boat making application. The same exterior glue as in all exterior construction grade plywoods is used. The main difference, and what makes marine plywood more expensive is the voidless inner plys that make up the plywood. Inner plys must be solid. (A boats motor causes vibration, if a plywood with inner voids is used the plywood can delaminate. Marine plywood has all voidless inner plys.)

Post by **Tim** » Fri Oct 26, 2007 5:54 pm

I used the cherry furniture ply with good success for non-structural interior components. None of the cherry pieces is of significant structural importance to the boat.

Keep your boat interior dry, and you can use almost anything that is of good quality. Furniture-grade veneer plywood, while using interior glues, tends to be of decent overall quality. Stay away from the junky plywood; it's pretty easy to tell what's decent and what's not. Voids are bad.

My choice for structural components would still be a marine grade plywood of your choosing, mostly for the additional laminations and void-free nature of the product.

suntreader · Post by **suntreader** » Fri Oct 26, 2007 7:36 pm

Thanks for the info. I've been around the plywood debates for years and have always heard to not use the interior grades, even in the interior, in the boat because of the interior glue not being waterproof. That was the theory I guess, but it looks like it has worked just fine in practice.

In theory there is not difference between practice and theory, but in practice there is.

I'll probably go with some birch ply with a mahogany stain followed by half a dozen coats of semi-gloss varnish.

Dave

jollyboat · Post by **jollyboat** » Fri Oct 26, 2007 9:38 pm

ok Dave - sounds like a plan - with that said I am going to edit my first reply post a bit, it is far to long -I will leave the link to the website that lists some interesting plywood info on for anyone interested

Rachel · Post by **Rachel** » Sat Oct 27, 2007 11:48 am

It's good to see you again Suntreader. I've wondered a few times if you were still working on your project and how it was going.

I suppose this might be a bit of a "look who's talking" comment, but it would be great to see some photos and commentary on what you've done so far. I remember you were tackling the "World's Oiliest Bilge" when you were last here. Sounds like you've gotten a lot done since then.

Rachel

suntreader · Post by **suntreader** » Mon Oct 29, 2007 9:55 am

Hey, nice to be remembered!

The project is humming along pretty well. The summer slowed things down a bit, I was working tweleve hour days and it was way too hot to be covered in fiberglass dust for my few hours of leisure. Now that the weather has cooled some and I have more time things are moving very quickly.

The engine is all torn down for rebuilding, need to find a friendly machine shop now; the interior is all ground/sanded and all the punky plywood is gone; still a few pieces of hardware to come off, but mostly done; the decks are sanded from the stern as far as the step in the cabin trunk. I figure we'll get the decks completely sanded this week, I'm off to Chicago from Wednesday through Sunday, then next week wash and scrub the whole thing to get it ready for expoxy and glass work, followed by interior rebuild, and then off to the races with finishes, systems, etc.

I'll post an update with pictures in the projects board.

Dave

Rachel · Post by **Rachel** » Mon Oct 29, 2007 12:58 pm

suntreader wrote:I'll post an update with pictures in the projects board.

Oh please do - sounds like you've been making some great progress!

Rachel