Im about to make my 2nd plywood board. I broke my first one yesterday. It was made out of a single sheet of 12mm marine ply and then painted with normal varnish. This time im thinking of making the board out of 9mm ply and then putting a layer of glass on both sides. Will this give me a stronger board than the 12mm ply only board? Im hoping that the thinner ply will offset the weight added by the glass.
I make all my wood boards 9mm thick, some of the first ones I made I covered both side with 6 oz. glass. Pretty flexy: a little too much for my taste. I broke one of them (i'm 85kg), but I think the main cause of the break was point loading from the binding plates I had mounted on it.
Now I form a 3-4mm concave into my boards, I find the concave really stiffens them up (much more than glass) and this makes them strong enough that they don't need any glass.
The choice of wood is important. Structural and weight characteristics vary tremendously by species. Okume (sp?) marine ply for example, is not very strong. Fir is ok but not too stable: when it gets damp, it tends to warp and check. If you glass it that shouldn't be a problem. I use birch.
At the very least seal your wood with a few coats of epoxy so that the water can't get in there. Pay particular attention to the end-grain.
I vacuum laminate my boards with thin layers on a rocker table with the concave shaped in.
Like I said I usually don't glass my wood boards anymore, If you're just going to glass one side, do the bottom. As far as I can tell wood boards only fail in tension, so that's where any extra strength would be needed. Plus with glass on the bottom you'll be able to hit sliders, etc. without chewing up the board.
I started with overbuilt designs and then started making them thinner/lighter. Beleive me, I have tried, but since I started using the concave I have only been able to break one board, and that was a very big one at 150x46cm so lots of leverage on the hard landings. I was kitelooping it lots and one time I came down really hot from around 10', just happend to land with the board flat on the back of a huge piece of Lake Superior chop. Board cracked under my back foot: typical. I think the board's failure kept my knees and/or ankles from exploding.
You are the second guy who asked me about flip tips this week. I don't understand why people want them, I think they are especially unnecessary on a really thin board. It would be possible with the method I use, but more work to the point where you might be better off using foam and glass. Maybe I will try making one this winter anyways.
How do you insure that the glass is watertight near the blindnuts?
How many layers of glass do you use?
Do you put a layer of epoxy down before you put the fiberglass on?
How bump-resistant is a glass and ply board. (commute by bike, will be strapping it to my bike.)
Just lay the glass on the plywood and apply the resin using a brush or roller. Let the the resin dry and then sand along the edges of your board to remove the glass that is hanging over the edges. Cutting the excess glass cloth off is not a good idea as the vibration messes up the bond between the wood and the glass. You only need one layer of glass.
Ya the board is pretty impact resistant and cos its so cheap to make another one throw it around as much as you want.
I used a polyurethane woodglue that foams up, and there were voids in between the layers. I didn't use a vacuum, maybe that would help - what do you use to pull the vacuum?
The vacuum pump is important, it gives you a lmainate without any voids and full contact of all the layers. I got my vacuum pump from e-bay, you can also use some airbrush compressors (it if has a threaded intake) or use a 'frige compressor.
The polyurethane glue is impressive stuff, I am not sure how water resistant it is but it seems plenty strong. If you coat the finished board and especially all the end-grain with a good sealer like epoxy it will probably be fine. Then again I am not sure polyurethane glue will cure in a vacuum.
The choice of the wood you use is very important. I use 3 layers 3mm, of a wood that is commonly sold as "baltic birch" which I beleive is imported from Russia. It is a fairly dense and strong wood: way stronger than say, "luan" plywood and even some of the commonly available marine plywoods. If the wood you are using is not as strong, you either have to use more (thicker) layers of wood or consider putting a layer of glass on the bottom and maybe even a layer of glass on the deck as well.