Exterior versus interior plywood grades

[nelsonrx]

Hi everyone,
I'm a noob to the site and have spent the last few weeks doing all of the reading that I can. After looking at all of the great designs I've decided that I ABSOLUTELY love the widget profile, so I will be building one over the next few months/year. After reading a lot about sidewall construction (and considering that I live in Florida where temperature extremes are usually not an issue) I've decided to go with solid-wall plywood construction for the sides. I plan on building a woodie, and thus using CPES and then a spar urethane for UV protection. My question comes around the grade of plywood that I'd like to use for the sides. Obviously exterior grade plywood would be the best bet, but could I get away with using cabinet grade interior plywood as long as the cut edges are heavily treated with CPES? Why use interior grade, you say? Well, I'd like to use a nice wood species plywood (i.e mahogany, cherry, etc) and I can't seem to find much in the exterior grade section for these. I believe the only difference between interior and exterior grade plywood is the glue used in the lamination process, but I'm thinking if I saturate the heck out of the faces and edges of the plywood with CPES that I should be okay. Any thoughts out there on this one?

[doug hodder]

It's no secret that I'm not a fan of CPE$, but the thing with it is....once you put it on....that's it, doesn't matter how much more you pile on in subsequent coats. It ain't gonna penetrate any more than it did on the first go around. It takes forever to really build up any depth on it for that nice "clear" finish that a woody ought to have. I've built 2 mahogany woodies and 2 others with epoxy coatings on them.

Epoxy will also seal like the CPE$, and you can really build it up to smooth out the sides. You also have the option to use additives to thicken the resin for different applications, whether it be filling, gluing etc... Epoxy will build much faster and give a deeper appearance to the overall project finish. CPE$....not worth it. Other opinions may vary to each their own. Doug

[Shadow Catcher]

The Issue should be exterior vs marine grade and I have seen it with the greatest veneers.

[kirkman]

Yes you can use cabinet grade plywood. I to am not a fan of CPES ether, you are much better off using a Epoxy like one form these guys http://www.mertons.com/index.html. A tear is not a boat but it is out in the weather like a boat and you will get much better protection from epoxy than CPES. Give them a call and they will hook you up with the right stuff......Just my 2 cents! You might want to read this article also before you use spar urethane.http://hardwoodlumberandmore.com/Articles/ArticleViewPage/tabid/75/ArticleId/26/Paint-On-A-Clear-Finish.aspx
Jason

[StPatron]

Quoted from the previous article: "The "spar" varnish (initially) looked fabulous; but, after about 2 weeks it began to develop small cracks. In rapid order the door began to turn black, started to mold and the smell was enough to knock a buzzard off of a manure wagon."

IMO, those remarks completely destroy the validity of the writer's "research". I have NEVER witnessed spar varnishes deteriorating to those conditions over a period of years... or the development of cracks in 2 weeks.

You might want to read these articles before using System Three products. IMO, there are much better products available.

http://www.oneoceankayaks.com/Epoxyhtm/epox12m.htm

http://www.oneoceankayaks.com/Epoxtest.htm

[working on it]

I am building my TTT using 3/4" (sanded to 23/32") interior cabinet grade plywood, so I'm really interested in WATERPROOFING my trailer.Even though I haven't reached the coating stage in my build, I've researched it for a long time. I've finally decided that I'll presoak and seal with the mineral spirits and cheap polyurethane mix much discussed here on this forum, then overcoat it with other oil based finishes: heat reflective paint on top, and an enamel on sides (both Alkyd resin based). In my trailer researches, and while putting in a wooden truck bed, I found a finish test that touched on both projects: http://www.mar-k.com/final_summary.html. I just thought I'd throw this in to muddy the waters. I've always painted my previous outdoor builds with rattle-can Krylon paints, with little or no prep, and some are still ok after years in the full exposure to sun and weather. Go figure ?!?

[Larry C]

It's no secret that I'm not a fan of CPE$, but the thing with it is....once you put it on....that's it, doesn't matter how much more you pile on in subsequent coats. It ain't gonna penetrate any more than it did on the first go around.

That's a great point, it will take several subsequent coats of this very thin product to build a smooth surface, where epoxy will do it much quicker, and is less expensive. Epoxy can be used for sealing, laminating fiberglass, making fillets, making gap fillers, as a glue etc. To me It just makes a whole lot more sense than using something that was intended primarily for repairing punky rotten wood.

[nelsonrx]

that's what I thought. I figured as long as you seal the edges and the faces with some sort of epoxy, then the interior versus exterior grade issue wouldn't matter. My plan is to build the widget and my main concern after reading a lot of posts is applying the epoxy once its vertical. I plan on doing the sides laid out flat, but does anyone have experience with epoxying the curved roof, especially the more vertical sections, and any drips/tips/etc?

[doug hodder]

I've done a lot of the really serious thickness build up while the sides were laying flat. I do however, always sand and do the final applications of epoxy once they are vertical. If you use the correct roller, not a brush, and apply it carefully with tipping off after application, it is no problem.

The thing is...you can't just do it all in 1 or 2 applications. To do it correctly, it needs to be built up in a number of applications and I always sand between coats. It gives you a much flatter finish. I've never had a sag in the epoxy on the sides of mine while applying it vertically, but I take my time. I also wouldn't try to do it outside if it were me.

I've used West Systems on all my builds so am only familiar with it's application. I'm unsure just how the viscosity of the other brands may be which will determine it's "sagability". West hangs on the vertical just fine for me. I only mix up like 8 or 9 pumps of material and work that out, then make up another batch. Once you catalyze it, the clock is running, so don't get too much mixed up that you can't deal with.

Read the online application information. If you follow the instructions, it's easy. While some suggest that it can be thinned, I don't do that, or would recommend it. It works fine just as it was designed. If you cut a corner and have a problem. It can be a real headache to correct. Others opinions may vary. Doug

As a side note....epoxy will not work well on a very hard 90 degree edge so ease your edges.
Don't use an oil based stain, an aniline dye either water or alcohol based is preferred.

[Larry C]

The thing is...you can't just do it all in 1 or 2 applications. To do it correctly, it needs to be built up in a number of applications and I always sand between coats. It gives you a much flatter finish.

...Just to add to what Doug has written, each rolled on applications should be rolled out thin. If you warm the epoxy and have a warm shop to start helps a lot. As Doug said, sanding between each application is a must. I will add that using a scraper to remove any runs before sanding is a good idea.
The one thing I have seen done by some boat builders and also I have seen here is ignoring the sanding steps whether using just epoxy over wood or epoxy/glass over wood is not sanding the epoxy to a smooth finish, and just leaving the lumpy orange peel finish. Whether your final finish is paint or varnish, the end result will only be as smooth as underlying surface (epoxy).

Larry

[working on it]

Quoting myself:

I am building my TTT using 3/4" (sanded to 23/32") interior cabinet grade plywood, so I'm really interested in WATERPROOFING my trailer.Even though I haven't reached the coating stage in my build, I've researched it for a long time. I've finally decided that I'll presoak and seal with the mineral spirits and cheap polyurethane mix much discussed here on this forum, then overcoat it with other oil based finishes: heat reflective paint on top, and an enamel on sides (both Alkyd resin based)

.... I just finished a test panel (to eventually become the galley shelf, later to be covered with laminate anyway). I started with the pre-sanded (both sides) cabinet grade 3/4" plywood, coated with two coats of 50/50 polyurethane and allowed to dry a week. Then two coats of 100% polyurethane, and dried a week. Then two coats of Tractor Supply (Valspar) Farm and Implement Enamel (Massey-Ferguson Gray), dried for 24 hours. With each step, the grain of the wood became more pronounced, so much so that I'm concerned about the problem of keeping the surface clean on the road...so rough it'll hold on to dirt. Conversely, the other side, uncoated at all, is still smooth. My wife says it should be sanded between each coat, but I'm thinking that sanding with 150-220 grit between the enamel topcoats would be enough. I want all the poly to remain for waterproofing, and for the initial topcoat to have enough tooth to grab onto, so the final coat will go on the re-sanded topcoat base. Remember, I'm a rattle-can painter, and never on wood that I cared about 'til now.

[doug hodder]

Seeings how you work on older cars...do you have a compressor? If so..pick up an inexpensive gun and try your hand at doing some paint work, practice on a trash can or a sheet of hardboard (cheap). You can get a quart of alkyd enamel for less than 20$ and just use some acetone for a reducer. I do put a shot of gloss hardener in it though. How many rattle cans do you estimate it's going to take and what's the cost? It's much quicker/easier to evenly coat it and keep a wet edge over a rattle can. You won't learn any younger. Just a thought. I don't balk a the price of a new tool. My spray guns have paid for themselves many times over. Doug

[working on it]

Thanks for your advice, Doug...I had composed a long reply, then lost my connection so I'll make it short(er). I have neither the skill nor painting conditions nor proper equipment to spray gun the TTT. My rattle-can approach was only successful on non-critical surfaces. As many others here have done before me, I was wanting to use brush or roller to paint the plywood. I am reasonably sure that no one will mistake my work for that of an accomplished craftsman, as many of you here are, but would be a novice's best first effort. I'm a shadetree mechanic, now a shadetree builder...and happy to admit my mistakes when I make them. Sure, I'd like to spray it on, but I know I can't control the dust and overspray in my shed/garage over a large surface (I live on a sandy hill, with dogs in the yard stirring up dust). But I might be able to control the conditions over a smaller area, clean-then brush or roller-then cover, then moving on to another section. If I could afford it, I'd have a pro do it, but then I couldn't say that I did it. The rest of the trailer is a personal creation, though helped by my friend with his better equipment and prior experience, but from now on it's up to me alone. What I need to know now, is if I have to sand between every coat, or just before the final topcoat. If it doesn't turn out perfect, I'll try again.