Things you need to know about marine plywood

Discussion in 'Boat Modification Journal' started by Mark J, Jun 18, 2009.

  1. Mark J

    Mark J New Member

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    Four Oaks, NC
    The most important thing to know is that it isn't some magical wood that doesn't rot. In most cases it actually isnt a very durable wood when it comes to rot. It is more susceptible to rot then domestic woods like southern yellow pine.

    Second most important thing to know is it is NOT treated with anything to keep it from rotting.

    So what makes it marine plywood?

    It's laminations. They are numerous, virtually void free, it's generally lighter in weight. Some cases, alot lighter.

    It's strength. It has a high shear strength. These panels are solid panels.
    The glue is a waterproof glue. You can boil a piece of marine plywood and then freeze it and it doesn't delaminate.

    Nice little fact to know. I've thrown pine plywood scraps out in the scrap pile alongside marine plywood.
    The marine plywood won't last near as long UNLESS you properly coat it with the proper product and it will outlast you.

    If you aren't going to spend the money for epoxy don't waste the money on marine plywood. You'll be disappointed with the end results.
     
  2. bcheney

    bcheney New Member

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    mississippi
    Thank you for the knowledge. I have a boat that needs new decking and have been going back and forth on the matter. Do you have a suggestion on a coating for treated plywood. I do want to go through the effort of re decking and carpeting the boat to have to redo it again in three years, but i don't thing three gallons of epoxy is in the budget either.
     

  3. Mark J

    Mark J New Member

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    Nothing I would recommend based on the fact I don't use it.
    I woulduse exterior and epoxy before I would use treated.
     
  4. ShilohRed

    ShilohRed New Member

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    4,339
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    Mark years ago when all we used were home made john boats. We used copperseal on the plywood. Made it last a long time.
    But the last 10 years we went to just standard Exterior plywood and its out lasted the High dollar stuff.
    But your right people think its going to last 30 years without help and its not going to do it.
    pete
     
  5. Mark J

    Mark J New Member

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    Pete, I think my next build will be that Phantom 22.:big_smile:
    Getting my sanding arms and longboards ready.
     
  6. ShilohRed

    ShilohRed New Member

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    4,339
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    Mark that will be one fine boat for sure. I know when you get it done, it will sure show people what you can do building your own boat.
    pete
     
  7. metalman

    metalman Well-Known Member

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    3,447
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    Winston
    Good post Mark.
    The term "Marine Plywood" is probably one of the most misunderstood terms in boating or carpentry...W
     
  8. bcheney

    bcheney New Member

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    State:
    mississippi
    Thanks everyone. Do you think that exterior grade luan would be an acceptable choice then? I'm not looking for a 30 year cure here, I just don't want to be like my friends and say "I just put new wood on last year and it's already falling apart"
     
  9. Mark J

    Mark J New Member

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    9,407
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    I built 2 canoes from exterior 5.2mm luan plywood.
    I'll never do it again. It's not real easy to work with.

    If it's a boat floor I'd go with exterior pine plywood epoxy and glass if I didn't go with Meranti marine plywood.

    What needs to be realized is that most of these marine plywoods excluding fir come from offshore. They originate from tropical trees and are not rot resistant but they are lighter. It's a trade off. Rot resistance and weight.
    Even though they are not resistant, with using the proper materials you can make them last indefinately with some care.

    Boats are like planes. You need to be mindful of weight.
    It's not hard to turn a good running boat into a dog.
     
  10. rasmotherman

    rasmotherman New Member

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    State:
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    mark, can you just put epoxy down or do you have to glass and epoxy. can you briefly explain the steps, my boat has a rather large deck and doing both may set it back awhile longer. i can get 1/2" treated for free. would it be possible to throw some heavy coats on it to keep it from reacting with the aluminum. right now i am just not walking on the spongy areas. i need a remedy soon and as you know, i have about a $0 budget
     
  11. Mark J

    Mark J New Member

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    I myself prefer to use glass if it's just 6oz cloth for durability and to prevent checking. Epoxy isn't armor plating. If you drop an anchor on it you might compromise it. If you drop a scuba tank on it it might compromise it. It's hard stuff but but like anything else, you hit it right, scrape it right, with the right object you'll scar it, crack it, or break it.

    Domestic plywoods like pine and fir will check (cracks in the surface). This includes treated plywood.
    It will check through ever how many coats of epoxy you want to put on it.
    When it does it exposes the interior plies to water damage.
    The ONLY way to keep it from checking is to glass the surface. It can't check then.

    If I went the treated route I would rack up that plywood for several months to dry it before it ever went in a boat. You would need to do that before you coated it with anything otherwise you trap the moisture and the weight in it.
     
  12. rasmotherman

    rasmotherman New Member

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    1,043
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    thanks. if i used the treated i may be able to save up for the epoxy and glass but it would be hard to do both. hopefully one of the 100 companies that have my resume call me soon
     
  13. Little Mac

    Little Mac Active Member

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    I made that mistake when I rebuilt my boat, I epoxied the heck out of it, but I did not lay down glass over the entire floor, just the seams. A mistake I now am living with, that wood checked/cracked badly, all that hard work and one mistake ruined it. Folks listen to Mark he knows his stuff, I guess I can sand it all down and glass it???? I dunno, I hate to rip it all out again:embarassed: I have been thinking of starting on a new boat that I have had the plans to for over a year. :smile2: Mark, what is your thoughts on this non blush epoxy they are running on ebay?
     
  14. Mark J

    Mark J New Member

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    9,407
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    Jimmy, yea you can still glass it.

    I haven't seen that epoxy on Ebay but the epoxy I use is non blushing and cheap. It actually has some better qualities and better wet out then an expensive System 3.
    Bateau sells it labeled as Marine Epoxy. 3 gallons for 155 bucks.
    I love using it. no complaints.
     
  15. Little Mac

    Little Mac Active Member

    Messages:
    1,828
    State:
    NW Arkansa
    Good, I figured the floor was ruined. What wieght glass would you recommend? That is a very good price on epoxy, This stuff on ebay is about 68 a gallon. I have had batuea bookmarked for a few years, I will go look it up. Thanks Bud...... Mac
     
  16. Mark J

    Mark J New Member

    Messages:
    9,407
    State:
    Four Oaks, NC
    6 oz cloth is what I use and easy to wet out. If it's high traffic and takes alot of abuse lay a second layer of 6oz right on top of it.

    Fill those checks with some wood flour thickened epoxy.
    You really dont need to fair it all out. Just roll on a couple of coats of epoxy then put some kiwigrip, interdeck, or whatever other flavor of anti skid you want.

    That Marine Epoxy is actually better sutied to laminating glass then the System 3 in my opinion. It just flows and wets out so much better. More viscous.
     
  17. rasmotherman

    rasmotherman New Member

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    1,043
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    i know an easier way..what do you charge an hour mark?
     
  18. Mark J

    Mark J New Member

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    Four Oaks, NC
    Alot more then the cost of epoxy and glass:big_smile:

    Typical 18 foot hull I wouldn't replace the floor for less then 1500 and that is with no stringer damage. Aluminum boat? 10,000.
    I absolutely hate aluminum boats. Aint but so much you can do with one unless you know how to and have the equipment to weld aluminum and then it may not be possible.

    In my opinion, just a poor building material from several different angles.
    The only positive I can see is affordability on the purchase end.
     
  19. rasmotherman

    rasmotherman New Member

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    i love alum for the fact that if i punch a hole i can TIG it up or patch and TIG and i am back on the water. with fiberglass i dont know how to mess with it
     
  20. Mark J

    Mark J New Member

    Messages:
    9,407
    State:
    Four Oaks, NC
    You can TIG it as long as you can get it clean enough to TIG.