waterproofing plywood

Discussion in 'Wooden Boat Building and Restoration' started by Head2wind, Jun 16, 2015.

  1. Head2wind
    Joined: Jun 2015
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    Head2wind New Member

    In addition to a backyard fleet and a nice 30' Santana, I have an aluminum snowmobile trailer 8.5'x20' decked w/ 5 sheets of treated plywood. The original deck (CCA) lasted 10-12 yrs, but the replacement (CA) cracked and warped in one year. It lives in the sun. After being forewarned of the present treated plywood, I've invested in ABDouglas. My thought is to epoxy the sheets top and bottom with perhaps CPES or suitable paints. I don't want to do this repeatedly. The trailer is in good shape; now if I could keep a deck on it... I'd appreciate your thoughts before purchasing a mistake.

    thanks
     
  2. Petros
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    Petros Senior Member

    Epoxy is tough but it is not as flexible as paint and tend to crack, and it is not UV resistant, so you have to put up to seven layers of paint or UV protective finish over epoxy to protect it.

    Sunlight tends to break down both wood and most finishes. If moisture also penetrates the coating and traps it in the wood it tends to degrade the wood faster and hasten the cracking and peeling of the finish. So what ever you put on it must be UV resistant and you will have to expect to refinish it regularly to seal up the tiny cracks that form on the surface over time. One author recommends recoating clear finish every six months if you want it to stay sealed (that is over a seven layer base!).

    OTOH 10 to 12 years is not so bad for a wood surface exposed to the weather, the alternative is to just expect to replace it every decade.

    Using the treated wood is a good idea, if it was me I would seal it, particularly the end grain, and than put 5 to 7 coats of polyeurethane or oil based paint on it, perhaps using the "non-skid" grit into the top two coats on the top side. And than install it, filling all of the fastener holes with some kind of weather proof sealant, before I installed the screws. It is known as installing the fasteners "wet", so it seals around each fastener head. I think I would skip the epoxy.
     
  3. rasorinc
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    rasorinc Senior Member

    I spent 35 years building both commercial and residential in the western foothills of the Sierra Nevada mts north of Sacramento and all the grading guys used Piss Fir on their trailers. No grain on their boards almost looks like pressed wood but they swear that it lasts a long time. Don't try to burn it as it sparks and pops.
    No way you use it in construction it is junk wood and cheap. They never treated it. http://enc.tfode.com/Piss_Fir
     
  4. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I like that idea of using rough untreated lumber. Might add 300 pounds if using 1.5" decking instead of 0.75" plywood, but it should be cheaper and when you are done with it you can dispose of it cheaper than something treated.
     
  5. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You might consider a sacrificial layer of plywood over plywood or solid PT. Let it take the beating, have it's finish gouged and abated, then, when it's time, yank it off, knowing the stuff below is still in pretty solid shape and reskin it again.
     
  6. Petros
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    Petros Senior Member

    PAR brings up a good point. Using 1.5" pressure treated plank will last longer than plywood, and likely cost less too. the sawn lumber will also weight less than plywood, so it will not add as much weight as using 1.5" plywood over .75" plywood.

    I have seen solid sawn fir, untreated, last for almost 20 years in the wet Pacific North West. If it was pressure treated, it should hold up even better.
     
  7. PAR
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    PAR Yacht Designer/Builder

    Using rough cut stock will rot much faster than well smoothed material, simply from having more surface area for things to grow, cling, etc. CA treated stock will perform better then untreated, but you need to be careful about the quality of the PT stock, which tends to be poor to modest at best in many big box stores.

    If using untreated stock, soak the crap out of it with kerosine for a week, before you paint it. Let this dry good, then a moderate diluted first coat of oil based primer, followed with 3 or more thin coats of straight or only slightly thinned primer. Again, with top coats use multiple thin coats, instead of 2 regular coats. It'll stick better and last longer.
     
  8. Head2wind
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    Head2wind New Member

    I read the Piss Fir piece. I'll keep that in files - sounds like something useful for a variety of jobs

    I suspect I won't find much of it out here on the Maryland Eastern Shore
     
  9. Head2wind
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    Head2wind New Member

    The trailer was designed to hold 1/2" plywood down in measured recesses. The crossbeams are 32" apart. The original ply did a fine job over the years, but after rejecting polyethylene sheets (noodles in the sun), aluminum planking (no one makes 1/2" versions and frighteningly expensive).

    I did find this tidbit on the 'PaintTalk' forum. It sounds affordable and to the point.

    http://inspectapedia.com/BestPractices/Paints_Stains_Exterior.htm#Application

    "Use of Sealers on New Wood Before Painting or Staining - Water Repellent Preservatives (WRPs)

    For the best protection of the underlying wood and the longest lasting finishes, bare wood should be sealed with a water-repellent preservative (WRP) before priming and painting or staining. WRPs contain a small amount of wax or other water repellent and a mildewcide, fungicide, or both, usually in a solvent base. The preservatives help prevent mildew and decay in above-ground applications but are not meant for ground contact. Some WRPs contain UV blockers as well, which slow down the degradation of the outer wood fibers.

    While sometimes formulated as a finish treatment for siding, some WRPs can be used as a pretreatment for painting and are recommended for that use by the USDA Forest Products Laboratory (FPL) and Western Wood Products Association (WWPA). Research shows that WRPs resist water entry better than acrylic primers. On bevel siding, they also reduce warping, splitting, and mildew growth. They can also improve paint performance on hard-to-paint woods, such as southern yellow pine and Douglas fir.

    In new construction, the FPL recommends that siding and trim be coated on all sides with a paintable WRP such as DAP Woodlife® or Cuprinol’s Clear Wood Preservative, preferably by dipping or with a brush, roller, or pad. If the siding or trim is already installed, they suggest treating all places vulnerable to water entry, including door bottoms, window sills, lap and butt joints, edges and ends of trim, and any end grain on panel products such as plywood sidings.

    If used as a pretreatment for paint, apply to bare, dry wood when it is above 50°F, and use only a single coat or excess wax buildup on the surface could affect the paint adhesion. Allow two days of warm weather to dry, or up to a week if the material was dipped. If painted before the solvent has evaporated and the wax absorbed, the paint can be discolored and not bond well."
     
  10. rasorinc
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    rasorinc Senior Member

  11. PAR
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    PAR Yacht Designer/Builder

    The data you've found is a bit misleading, as all coatings can benefit from a pretreatment of some sort, be this a bond promoter, a sealer, a tie coat, whatever. This statistical data gathered up by the likes of the USFS and other agencies, is a bit like the medical studies done on medications and procedures. Some will say one thing, while others something else and they're only valuable when looked at in context, in this case the whole range and set of studies and tests, rather than one that says what someone wants to hear. In other words, take it with a grain of salt and a WRP coating isn't as good as a WBP coating and what are the WRP's anyway (most have wax in them, which really screws with subsequent top coats BTW).

    For example, Thompsons deck coating stuff is well known and lots of decks have it on them. Ever try to paint over a deck with Thompsons on it? Yep, once you go Thompsons, you're always going Thompsons, because most everything else doesn't stick. This is the classic conundrum of these coatings (WRP's repel more than just water). Enter the WBP coatings and adhesives, such as that used in marine and some exterior grade plywoods and other "sheet" goods. These (if they remain intact, which is the key) actually prevent moisture from passing the barrier or glue line, rather than just resist or repel it. Simply put, it's a complex subject, but you have a number of options.

    Given your budget restraints, some of the fancy stuff, possibly including WRP's and epoxy are out. CA treated stock can be sealed up really well, if it's dry to start with, though the usual problem is the stuff is saturated, when you find it in the big box store, so coatings don't stick worth a damn. Finding dry PT is hard, so you might consider drying it yourself. The easy way is to rent a storage shed, like those mini storage things, popping up on every major road. Sticker the wood up and place a $15 Wal-Mart fan at one end to blow across and through the stickered stack. Leave it there for a month and it'll be pretty darn dry when you pull it out.

    Once dry, just use plain old school prep and painting practices, as I've described above (previous post) with as good an oil based paint as you can afford. This is the low budget way and if you've ever torn down or repaired a 100 year old house, where these old school techniques where employed, you'll see how well they work.
     
  12. Head2wind
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    Head2wind New Member

    I greatly appreciate your feedback. Trying to fight thru the bogus or biased advertising makes it difficult for everyone. I wanted to go to the supplier and pick up my plywood order today, but like much of the country, it's likely to get soaked with all the rain recently. I'll be able to see soggy new plywood in my dreams.
     
  13. goodwilltoall
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    goodwilltoall Senior Member

    Greetings,

    I have been using one coat of epoxy on boat building project until there is time to do the complete fiberglass coating, it has held up very well these past few years with common plywood. There a few small keel bulkheads missed and you could see it darken a little so went ahead and hit those.

    One coat epoxy on plywood will keep most moisture from penetrating but still allow evaporation. As far as flexibility use the 1-1 or 2-1 mixes. This was something I had wanted just that reason. Mix I use is usually 2-1 but you can go 1-1 which will give it much more flexibility and think it increases water resistance as well. Some type of UV protection is required.
     
  14. goodwilltoall
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    goodwilltoall Senior Member

    If using wood boards, use the air dried rather than kilned as that destroys the cell structure and then its awful at coping with moisture gain/loss. We have had air dried yellow pine and fir planking for at least 15 years that's still very stiff even being outdoors their entire use. The northern fir (even knotty) is an excellent long lasting wood, light weight (laborers love it), and cheap. My brother put about 80 pcs. under a tarp for a year or two and it destroyed them but as long as they can breath it excellent wood. You can buy kilned yellow pine at the big box stores but that stuff would have been burned a long time ago.

    As a side note, we were able to buy 2x10 - 16' fir planks for $15.00 in 2000 and would do it again but the distributer will not sell them any more because of OSHA. They even stopped selling the air dried yellow pine and most of the stuff comes up here from North Carolina. You can buy it there for about $20-25.00 a piece but once here sells for $48.00 (it was $42.00 in 2000) which is the cost of doing business but I despise it being kiln dried now.
     

  15. PAR
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    PAR Yacht Designer/Builder

    Nothing personal Goodwill, but your advise is full of mistruths and plain misunderstandings.

    The epoxy molecule is pretty big, which is why you need a minimum of a 10 mil coating to insure waterproofness. A thinner coating isn't going to do much, in terms of resistance, again, because of the physical size of the molecule and though some protection will be provided, the moisture gain/lose rate will remain the same. No formulator or industry professional would suggest your recommendations, in spite of the seemingly good luck you might have experienced.

    Kiln drying doesn't destroy the cellular structure of the wood. It's true naturally seasoned lumber is preferred for some structural elements, but moisture lose, be it natural or forced doesn't change wood, just it's moisture content. If kiln drying is done at too high a temperature or too quickly, then some damage can occur, but the cells aren't destroyed, just deformed a bit, typically near the surface, which can be milled off.

    The successes and failure of your pine and fir are simply species, care and cut related. Of course an unstickered stack will develop rot very quickly and this is true of most all species.
     
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