Building a raft, looking for advice.

I'm making a raft kinda like this one:

http://www.boatdesign.net/forums/showthread.php?t=16944

...except nicer

But I have some questions. I'd LOVE to use marine grade ply... but the local Home Depot doesn't have any. Current plan involves decent 3/4in ply - not the really cheap stuff that doesn't look like it'd be able to survive a brief rainstorm - and some non-pressure treated 2x4s (I could upgrade to pressure treated, but I'd rather not). Problem is sealing the wood and helping prevent rot... I'd rather not have to paint it (as in, I want to keep it fairly natural looking), and I'd like to keep the cost down fairly low.

Some looking around through here and on google has got me:

Borax for anti-rot
http://www.hares.net/boat/waterproofing.htm for sealing.

Any comment/advice/tips would be greatly appreciated

 

I don't know about the plywood, I'd use exterior grade. I'm not sure about the application or floor joist support so I won't comment about that.

You're right about not using pressure-treated. Not only is it heavy and more expensive, but it warps like crazy when the sun gets on it. All wood will warp, though. If it gets wet and water-logged, then dries in the sun, the top-side will dry (and shrink) before the bottom side, and for some reason the bottom side doesn't shrink enough to compensate for the other side, and you end up with warped wood.

So keep your wood DRY. The best way to do that is to paint it. If you want a natural look, go with something clear.

I have not tried anything other than polyurethane. Well, I did try Thompsons's water sealer but ... I don't want to get sued so I won't comment about it. Minwax Polyurethane works good enough for me. On the deck where there's lots of foot traffic (and you always have little rocks and gritty stuff stuck in your shoes, lawn chair legs, coolers and deep cycle batteries sliding around, etc.) you have to keep adding more coats as needed if you plan on this thing lasting more than a year or two.

On my first raft, I built them outdoors, didn't sand anything, and let the polyurethane dry in direct sunlight. They looked good but it quickly started cracking and peeling. I just made new ones this summer, and here's what I did.

I started with new lumber, never let it get wet. I built them indoors, out of the elements. I sanded the wood first with a palm sander, with a blow gun in my other hand to blow away the sawdust. I didn't sand it forever, I just got it so it felt significantly smoother than the un-sanded areas. That doesn't take long. Then after I put the polyurethane on with a brush, I let it dry for about an hour and sanded it by hand (no palm sander) using 150 grit. It doesn't take long to get a smooth feel under your fingertips. It's not hardwood, so don't expect it to feel like your $12,000 kitchen cabinets. After 3 or 4 coats, I let them dry in the barn for 2 days. (It was raining) Now I have these rafts and the water beads up like on a freshly waxed car. I wash them with the hose and an old rag after every trip, and I store them in the barn. One day I used the rafts to transport several thousand pounds of flag-stone to use as rip-rap to stop erosion on our camping island, and I scratched the hell out of the top plywood pretty bad, so I added another coat of polyurethane and it's good to go, except for a few chips and dents.