Marine or exterior grade

MallRat

Seaman Apprentice
Joined
Aug 7, 2018
Messages
46
Have acquired a '87 Sweetwater 16' pontoon. The 12' x 6' deck is weak and needs replaced.
I've sourced both Marine ABX and Exterior ACX plywood about a 4hr round trip drive from home.
The Marine ply is Douglas Fir and $79. The Exterior ply is Pine and $42. Both are 7 ply.

The boat will be on a trailer stored outside exposed to weather when not on the water. Will make a tarp to secure over the boat once all is complete.

I'm really leaning toward the more affordable Exterior ply for $150 vs the more expensive Marine ply. $240 is half the price I paid for the pontoon.

What life span should I expect from the Marine and Exterior plywoods.

From what I've read the is some debate over the weather proofing the plywood.
Some say to apply sides, top AND BOTTOM.
Some say to apply to sides and top only, not the bottom.

Leaning toward Not doing the bottom.

As far as waterproofing, plan on 1 part polyester resin and catalyst with 5 part acetone, apply and let dry, then repeat using less acetone each time until the final coat of just polyester resin. then sand and a final coat of epoxy resin. then sand and carpet.

Any water splashed up from lake can drain off bottom of the plywood while at the house and rain/snow can do the same.
I will turn this into a build post when I get started. prob not until spring.

Thoughts?
 

fishrdan

Admiral
Joined
Jan 25, 2008
Messages
6,989
Look for "pontoon decking", it's treated plywood and doesn't need any type of sealant to keep the wood from rotting, cut, install and enjoy your pontoon. Some outdoor stores can order treated marine plywood, Gander Mountain used to carry XL something or another.

Both doug fir (untreated/normal) marine plywood and exterior plywood will rot, since they are not treated. Sealing them is an extra step, more cost and labor. I wouldn't use poly resin as it's weak without the use of fiberglass, it will crack. You can use epoxy resin without the use of fiberglass, but that's more expense.

Check the cost of pontoon decking vs what it will cost for normal plywood with the added expense of sealing it. Might be cheaper with the expensive plywood.

FWIW, "marine" doug fir plywood has several layers laid at 45* (instead of all plys of normal plywood laid at 90*) and it also has all layers boat patches and virtually void free. But, it will still rot like exterior plywood.
 

JimS123

Fleet Admiral
Joined
Jul 27, 2007
Messages
7,952
True, both will rot. Exterior grade is really not intended to get wet, because it will be covered up or painted. Getting Marine grade wet is inevitable.

Marine grade will not delaminate and thus lose its strength like exterior grade. If it were my boat and i intended to keep it I wouldn't consider anything but marine grade. And then I would cover it with nautolex vinyl decking to make it last even longer.
 

JASinIL2006

Vice Admiral
Joined
Feb 10, 2012
Messages
5,524
True, both will rot. Exterior grade is really not intended to get wet, because it will be covered up or painted. Getting Marine grade wet is inevitable.

Marine grade will not delaminate and thus lose its strength like exterior grade. If it were my boat and i intended to keep it I wouldn't consider anything but marine grade. And then I would cover it with nautolex vinyl decking to make it last even longer.

I thought the big difference in marine grade vs. exterior grade plywood was the species of wood (Douglas-fir or Western Larch) and the absence of voids in the plies of marine grade. Otherwise, everything I've read indicates that both use waterproof glue that survives soaking and boiling tests. It seems like the only way exterior grade would delaminate more quickly would be if the wood itself rotted (which would be more likely due to the voids in exterior grade), but that's really a bigger issue than delamination.
 

JimS123

Fleet Admiral
Joined
Jul 27, 2007
Messages
7,952
Marine grade is good thru and thru. Exterior ACX is "good one side", which means that the back side can have voids and patches.

It probably doesn't really matter. The typical owner doesn't keep the boat for more than a few years, so it's the guy that buys the used boat that has the problems.
 

harringtondav

Commander
Joined
May 26, 2018
Messages
2,438
Most of what I know of hull and transom restoration is from watching Frisco Boaters Sea Ray resto. I'm pretty sure he didn't invent the techniques. Lots 'o research. He sealed his marine ply transom with epoxy before installation. Costly, but a long term fix.
 

Ned L

Commander
Joined
Sep 17, 2008
Messages
2,266
Yes, the biggest difference between marine ply and exterior ply is that marine ply has no internal voids (they are filled). They both use equal waterproof glue. If it were me I would use the exterior.
 

Blind Date

Chief Petty Officer
Joined
Mar 5, 2014
Messages
462
It probably doesn't really matter. The typical owner doesn't keep the boat for more than a few years, so it's the guy that buys the used boat that has the problems.

Yep and all the seating will need replacing well before the floor starts to get soft. Use ACX and seal it with 50/50 mixture of acetone and resin.
 

MallRat

Seaman Apprentice
Joined
Aug 7, 2018
Messages
46
Was going to add some pics but can't get them to upload. Will keep trying.

I have got everything off the toon. Did find out that the toons are aluminum and the floor joist are galvanized.

Too late but just wondering if I could have gotten the PT plywood since the floor joists are galvanized and the PT would not have contacted the aluminum directly?
 

ahicks

Captain
Joined
Sep 16, 2013
Messages
3,957
I'd go with the marine and know it's been done right, without cutting any corners. With luck, it will outlast you.

Also, rather than carpet to cover the floor, if you haven't looked into vinyl yet, most that have used it LOVE it because of it's ease of maintenance. A LOT of the new boats are coming with it.
 

MallRat

Seaman Apprentice
Joined
Aug 7, 2018
Messages
46
I don't know what the duck is going on here with the pic upload so to yell with it I QUIT
 

MallRat

Seaman Apprentice
Joined
Aug 7, 2018
Messages
46
I already have plywood. Was just going to use this as a build thread as well.

That was the reason i said it was too late to wonder or worry about the galvanized floor joist.

Was just trying to upload pics and could not get them up. I read the tutorial resized less than 700 kb. Even tried to email to my phone to upload. That didn't work. Tried to send to my dropbox and upload that way. Was still a no go. Keep getting php disabled error.

Was able to upload pics to a different post a couple days ago so i have no ideal what is going on. Don't think it is on my side now. That is why i said to yell with it.

By the way the plywood I ended up with was a 7 ply bcx that I'm putting spar urethane on top (c side) and the edges. Going to put on 3-5 coats. Then put my carpet on top of that. I figure since the boat is on trailer most of it life. When the ply gets wet it can drip dry under the tarp in yard.

Got stainless carriage bolts, washers, and nylon nuts to hold down the ply.
 
Top