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fat guy in a little boat
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So I'm gonna try to use a sheet of 3/4 marine grade plywood for a floor board on my 16foot Hyside. With the cooler, water jugs, kitchen box and rocket boxes on it, I figured it'd be stronger than those plastic sheets....
But. Should I coat it with something??
My last floor was just cheapo plywood, and it only lasted a couple trips.
Is Marine plywood good enough on its own, or will it rot and de laminate too??
Should I coat it in urethane??
I assume things like Thompsons Water Sealent for decks would be bad for the rubber...
Maybe just linseed oil??

What do you guys think?? Anyone else using a Marine plywood floor??

Thanks for any input I can get!!
 

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I painted mine with a couple of coats of deck/porch/patio paint from Lowe's, has held up great for a couple of seasons now.
 

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I have a plywood casting deck that I applied a few coats of polyurethane. Had no problems when I stored the deck in the garage. But when stored outside for two months, the polyurethane cracked, splintered and began peeling off (UV and rain damage). Your mileage may vary.

I like your linseed oil idea.
 

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I have built several plywood floors and can share my experience...

1. Plywood comes in lots of grades. Marine grade is great, but you could get by with an exterior rated XCD or XAC
(X = exterior glue A,B,C,D = is the grade of surface finish, A grade is knot free, B grade has knots cut out and filled with 'footballs' D grade will have exposed knots and voids in the layers) 3/4" thick is good, I tried some at 5/8" and was too bouncy. Don't use pressure treated plywood, the treated wood won't help and the splinters are toxic.

Here in Colorado the marine grade is not commonly stocked. You will probably need to special order it. Not all marine grade is rated for exterior use.

2. Exterior deck/patio paint will out perform spar varnish or polyurethane varnish for all wet contact applications. Linseed oil will not work well underwater.

3. Seal the edges and voids before painting. The exposed grain at the edges, knots, and voids will suck water deep into the plywood. Seal these with 2 ton epoxy thinned with denatured alcohol (thin to the consistency of cream) I like to give one coat to all exposed edges, then a second coat over the whole board.
2 part wood filler or auto body filler is good for filling voids.

Note: epoxy alone has poor UV resistance. This can be corrected by over painting with deck/patio paint. Or by adding UV blocking pigment to the epoxy.

I have some deck boards made of 3/4" XAC plywood that are still going strong after 5 years.
 

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This!!!

I have built several plywood floors and can share my experience...

1. Plywood comes in lots of grades. Marine grade is great, but you could get by with an exterior rated XCD or XAC
(X = exterior glue A,B,C,D = is the grade of surface finish, A grade is knot free, B grade has knots cut out and filled with 'footballs' D grade will have exposed knots and voids in the layers) 3/4" thick is good, I tried some at 5/8" and was too bouncy. Don't use pressure treated plywood, the treated wood won't help and the splinters are toxic.

Here in Colorado the marine grade is not commonly stocked. You will probably need to special order it. Not all marine grade is rated for exterior use.

2. Exterior deck/patio paint will out perform spar varnish or polyurethane varnish for all wet contact applications. Linseed oil will not work well underwater.

3. Seal the edges and voids before painting. The exposed grain at the edges, knots, and voids will suck water deep into the plywood. Seal these with 2 ton epoxy thinned with denatured alcohol (thin to the consistency of cream) I like to give one coat to all exposed edges, then a second coat over the whole board.
2 part wood filler or auto body filler is good for filling voids.

Note: epoxy alone has poor UV resistance. This can be corrected by over painting with deck/patio paint. Or by adding UV blocking pigment to the epoxy.

I have some deck boards made of 3/4" XAC plywood that are still going strong after 5 years.
Pretty much how I do it. I have had floors last 12+ years. Light sanding and retouching with deck paint every year, where needed.
 

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I use the best 3/4” plywood I can find and cover it with spray on truck bed liner. I had one floor in my foot well go bad and have to be replaced after two years. The rear beaver board and the replacement foot well floor are going strong after 6 and 4 years respectively with no maintenance.

Kyle
 

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Use the lumberyard the larger contractors use. Ask for 3/4" MDO form ply.
If they don't stock it, they should be able to get it from their distributor.

Alternatively, ask an old-school sign shop where they'd get it.
 

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I was able to contact Intermountain Wood Products, and they can order it in.
Price is $89 for a 3/4" 4x8 sheet
and $75 for a 1/2" 4x8 sheet

I am making my captain's floor and my beaver board from them for my 16
and then floor, side decks, and beaver for my MiniMax
 

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I've used oil based porch paint on my plywood trailer deck...you need to use a primer first but lasts well.....I've used spar varnish (oil based) on my plywood casting deck/floor also lasts well (I haven't used a primer but needs several coats over bare plywood) but I do another coat almost every year .....both would be good options in my opinion (my brother is a chemist who suggested the porch paint for the trailer surface).....but for a plywood floor I'd probably pick/do the spar varnish....Cheers, Chet
 

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If you use oil-based paints, try some alkyd hardener. You'll likely find it in the auto body paint supply store, but you can use it with regular rustoleum/krylon/etc commercial/industrial enamel paints--you don't have to buy auto body paint.


https://www.krylonindustrial.com/product/alkyd-hardener-2/


Use 8:1 or 10:1 and thin slightly with naphtha. It will go on smoother, wet out nicely and generally flow out brush/roller marks. Dries to the touch faster, and cures harder. I use it on raft frames, hockey goals, trailers, my dory, ammo cans, propane tanks, oars...anything you'd use a rattle can or regular oil based paint on..but it makes it that much better.

I rarely use bomb cans anymore unless it's small/touchup as this system goes on so nicely and wears much harder.
 

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Just use MDO and skip all the sealing and maintaince..
Lots of people swear by MDO. I haven't, but have heard enough to do so next time.


I'd still want to seal the edges.
I certainly swear by it now. I used 1/2" for my side boards and 5/8" for the floors of my 146DD. All I did was cut it to fit, use a router to round the edges and cut holes for straps (also rounded) to hang the floor from the frame and secure boxes to the floor. No sealing or treatment done at all. It is holding up great. I have one floor that has probably seen 80-100 days or more and while it has scratches and has faded a little, is nowhere near wearing through. The side boards aren't far behind that number of days and the same. My only complaint is it kind of smells funky when it gets wet...but it gets better over time. I think its the glue they use to bind it all together.

When I bought my 16' Avon it had plywood floors front to back and one piece broke in half and I had to replace it. I decided to do the Marine Ply and Spar Varnish route for that (this was before I tried MDO). I did 4 coats over the whole thing. It started checking on the first trip out and just isn't holding up like I had hoped. Feels like it would take a lot of maintenance to keep it nice.

So yeah... I heavily prefer MDO. Its not cheap, but its also not crazy expensive either. Similar in price to the Marine Ply I've found.


Does anybody know where to source 3/4" MDO in Grand Junction or Denver?
There are a bunch of places in Denver that carry it. As someone said, anywhere that caters to the concrete industry have it since they use it to make forms when they pour concrete.

I was unlucky the last time I bought it in that all the places that normally carry it for conctruction use had all sold through and had it on backorder.

My go to for weird plywood you can't find anywhere else is Austin Hardwoods, which is where I bought the last chunk of MDO for about $99 as I recall. It was either 5/8" or 3/4" 4'x8' sheet. I think the place that sells it for concrete use(but was out of it) was closer to $60 a sheet when they have it.

The other stuff I got I got from a friend that works for an Art transportation company. They make their crates from it and had a ton of off cuts. Unfortunately, the company sold to new owners and they were a lot more stingy about that stuff and that supply dried up. Like others have said, call around to a few sign shops that do outdoor signage. They use it heavily, and might have an off cut that fits your needs or could either sell you a whole sheet or tell you where they get it.
 
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