Interior or Exterior plywood? and What finish to use?

[Salivanto]

I was going to make a wooden teardrop and paint it, but since I've managed to get my hands on some red oak, I've been thinking that I should make a woodie to let that beauty show through. I wasn't happy with the appearance of the plywood I've been seeing at the local "orange box", so I checked out one of the specialty wood shops in the area. When I mentioned what I wanted to use it for, he steered me away from the interesting assortment of "interior grade" plywoods toward the two "exterior" plywoods which he had: Marine Fir and Marine Okoume. The less expensive of the two is the fir. Still, it's more than twice the cost of the same size board of the "orange box" stuff and I'm still suffering from sticker shock.

So, my first question is whether it's a good idea to stick with exterior grade plywood.

My second question is whether the kind of "regular" plywood which you get at Home Depot is typically exterior grade.

The fir is fairly nice stuff and looks nice on both sides. I've been going back and forth as to whether I should use thinner plywood and have framed and insulated sides, or whether I should stick with 3/4" plywood only sides. I'm thinking that since the fir is good on both sides that it might be enough to tip the scale for me so that I'll use the thicker plywood but no framing. My estimate -- assuming I can use the framing boards I already own - is that an insulated or an uninsulated wall will cost the same to build - as far as materials go, so I'm still not sure.

Another thought I had was to buy cheaper wood and paint it with epoxy paint, and still trim it out with the red oak, which I'd have to finish another way. Is there such a thing as clear epoxy varnish which would look good next to white epoxy paint?

[madjack]

...generally speaking, most ply is made with exterior type glue unless otherwise stated...marine grade ply is exterior ply with all the voids filled, unless you are going for a specific look that can only be met with the marine ply it is simply not necessary. I am using cabinet(14ply) grade ply on the Alligator Tear in both 3/4(sides) and 1/2(cabinets/floor) and 1/4 "bendy"ply for the roof and ceiling
madjack

[Salivanto]

Is that true even if it's not going to be covered by aluminium?

[Brad Lustig]

Depending on how you finish it, red oak can rot pretty easily since it has open cells. I'm speaking from a boat building and outdoor furniture point of view so it might be fine for a tear. White oak is a lot more water proof and rot resistant. Red oak is pretty wood, though

[Salivanto]

Depending on how you finish it, red oak can rot pretty easily since it has open cells.

I'm starting to wonder if I'm going about this backwards. At one point I had decided to use basic plywood and to paint it. Then I got my hands on some (free) red oak so now I'm looking at more expensive plywood. I suspect that the difference in price of the plywood will be far more than the value of this red oak I was given.

I'm also wondering how durable it needs to be. My thinking at this point is that I will be garaging the tear when not actually traveling in it.

So, what is the right way to finish red oak so that it will be durable outside? Fill the pores with putty and use a quality finish? What kind?

[IraRat]

Sali (I'll still call you that)...

Do you have 4 X 8 sheet size oak? (If not, you can probably stop reading.)

There are a lot of sealants you can use, including polyrethane that will really protect the wood, but that has its drawbacks too. (It can get cloudy overt time, crack, etc.)

I'm using standard, thin 3/8" ply walls, framed on the inside and covered on the outside with EXPENSIVE birch paneling. ($30 a sheet!) I'm using marine grade varnish to protect it, and as you'll see in another of my posts, that varnish cost a fortune TOO! ($35 a quart.) Over time, UV rays break down most finishes, including varnishes, so that gorgeous shine/gloss you get disappears over time. (If it's a shine you want in the first place.) The stuff I bought is Pratt & Lambert Vitralite spar varnish, rated for like 25 years.

The trick is, am I stupid enough to varnish the sides without first seeing if I like the resulting color? No, not THAT stupid.

The raw birch is very light, so just yesterday, I took two pieces of scrap birch (I had already cut my side skins so I had scrap pieces), and on one, I applied a coat of Miniwax natural wood stain. On the other, just the varnish. Over the course of the next few weeks, I'm going to apply additional coats of varnish to each, sanding in-between, to see the different results.

I used fairly large pieces of scrap to be able to make a good judgment, and what I've seen in just one day is that even just one coat of varnish darkened and brought the grain out of the birch beautifully--enough to know that I don't think I'll go with the staining. I think that will just make it darker.

I'm sure I'll be satisfied with the results after the tests, but if not, I will gladly write off the expense and start over.

The exterior skins are too important to not get them the way you want--and too expensive and a pain in the a** to have to redo them! Since they're going to be your walls too, it's doubly important for you.

I would say forget oak for the insides, and go with conventional framing, insulation and thin interior skins. Mind you, I'm no big expert--just passong along what I've learned here. I'm also not a big "paint" fan, neither for the look or practicality.If you felt compelled to, you could always paint OVER stain and varnish in the future. Trying to reverse it the other way is next to impossible.

[Salivanto]

Do you have 4 X 8 sheet size oak? (If not, you can probably stop reading.)

The red oak is in the form of boards -- about 3/4" thick and 14' long. I'm not sure of the width. I think it's 14" or so, but much of the wood is damaged. I think I'd need to rip it down to 4 or 7 inches or so to cut off all the bad wood.

My thought was to use it as trim on the outside.

There are a lot of sealants you can use, including polyrethane that will really protect the wood, but that has its drawbacks too. (It can get cloudy overt time, crack, etc.)

My original thought -- way back when -- was to use the same kind of "spar-urethane" which I've used on some other projects. If that means that I'll need to store the trailer in the garage and touch up every few years, that's fine.

My preference is against paint, but I'm having a hard time justifying 80 bucks a sheet for wood that looks nice enough to let show through.

[Brad Lustig]

I don't know of any way to seal the oak to prevent rot. I've heard of people using it on their boat and even with it sealed with epoxy and fiberglass, it usually ends up rotting. You might want to try this experiment (I've never done it, but I think I remember seeing it in Wood magazine). You can take a short section of red oak and stick one end in water and actually blow through it and see bubbles come up. Also if you leave it in the water, it'll actually wick the water almost all the way up. Much higher than most woods. If it were me, I'd either use it in the interior or just use it on another project.

[doug hodder]

I'm also doing a woody, I went with the okume as I wanted the mohogany look and color like on an real woody car. If you are going to do a natural or "bright" finish on your wood, plywood won't take on the dye/stain evenly enough. It'll be way light and dark. Okume takes on finishes more evenly. About red oak. Typically white oak is used in marine construction for framing, I wouldn't use red, unless you completely epoxy encapsulate it. It's also not as structurally strong as white. The tannins in it will also corrode steel screws over time. It may be one of those things that it really doesn't matter. How much are you going to use it and if it is garaged it shouldn't be an issue.

I bought 6mm okume and laminated it to 1/2" exterior ply, rather than buying the full thickness in okume. Marine grade okume is solid okume in all the layers, not just a top laminate. That's what you're paying for, also they use fungicidal glues, at least in the ocean proof products.

If you wanted to side it in the oak, I would resaw it down and plane it to 1/4" or so or you will be adding a ton of weight. That's how I'm doing my maple trim on mine. Just my thoughts on it, I'm sure others may not agree with me. Doug Hodder

[Rocosil]

The principal difference between marine grade fir and hardwood plywood for exterior use; aside from appearance, of course; is that fir will check as it weathers, even if it is varnished, painted, or epoxied and then varnished or painted. Hardwood marine ply, and particularly tropical hardwood marine ply, will not. To keep fir ply (marine or CDX) from checking in exposed conditions it should be sheathed with fiberglass cloth set in epoxy. MDO ply, the road-sign stuff, gets around this problem, but you'd probably want to paint it rather than using a clear finish.
I've used red oak for exterior trim on boats in a salt water environment. Properly epoxy coated, bedded in an anti-fungal marine bedding compound, and given 6 - 8 coats of marine varnish it looks lovely and will stand up to the elements, but it does take a lot of maintenance, i.e, repair all nicks and scratches as soon as you spot them and a light sanding and two fresh coats once a year. For interior trim it should work fine with just two or three coats of varnish. It should also be all right for framing if epoxy treated and bedded in and anti-fungal compound -- really seal the end grain thoroughly -- but, IMHO, it would add too much weight to a TD.
BTW, this is the first sign of life from me on this forum. I know diddlysquat about TDs, but what a fantastic place this is to learn.

[doug hodder]

Hey Rocosil........welcome to the forum. I only have fresh water boat building experience, so it's good to hear from others with salt experience. Are you planning on a TD? There's lots of materials and skills that cross over.....BTW. I do cloth over my fir on my boats, good point made...Doug Hodder

[Rocosil]

Thanks, Doug, for the kind words of welcome. Yes, I'd like to build a TD, but I'm far from sure I can away with it. I've heard grumblings around here along the line of, "Haven't you got enough projects already? Aren't you getting a little old for that sort of stuff? Lots of things that need fixing around this place. A camper!!!!!!! -- you've never been camping in your life!!!!!!!" All of which is, of course, true, but after couple of decades of playing around with boats and canoes I got involved with refurbishing old Volvos (Note that I'm not saying "restoring"); among them a 1962 PV544. That's the one that looks like the unsanforized version of a cross between a 1941 Pontiac and a 1946 Ford. In fact, its lines are very similar to those of a Modern Cub TD, and it occurred to me that it would be really cool to go to vintage Volvo gatherings towing one of those, saving some motel fees while I'm at it. So, that's where I'm at now; reading to learn more and taking my temperature from time to time to determine how serious this bug-bite really is.
Cheers,
Bob

[IraRat]

So, that's where I'm at now; reading to learn more and taking my temperature from time to time to determine how serious this bug-bite really is.
Bob

Bob, just check out Mexican Tear's gallery here and look at that galley.

How can your wife argue with THAT!?

(Mine has been yelling at me about a front door I have to replace. Building a TD is great for building up an immunity to letting that kind of nagging bother you.)

[Brad Lustig]

Here's a link with some info on plywood and wood used for building the little Weekender sailboat.

http://www.messing-about.com/weekender/woodFAQ.html