epoxy over plywood

[gunsmith]

I am going to make my cabin sides from 3/4" plywood. I had planned on prefabricating the pannels and covering the outside with a layer of glass imbeded in epoxy. They are then going to be screwed to there respective framing with epoxy adheasive as well. Is there any reason why I could not simply use the epoxy without the glass to surface my plywood? Would the glass serve any purpose? My finish is going to be an epoxy paint on the outside and I want the finish to be smooth and hard.

[Bruce Hooke]

What kind of plywood are you using? Douglas Fir plywood will generally check if it is not covered with glass cloth. Epoxy alone won't keep it from checking. With better quality (non-douglas fir) plywood this should not be an issue, in which case glass cloth probably becomes much less necessary.

Do the plans specify glass cloth?

[mmd]

With plywood that thick, a single layer of 'glass serves mainly as an abrasion-resistant surface; it does not add any appreciable strength nor stabilzation. If there is nothing rubbing on the cabin such as running rigging, I'd just go with the epoxy coat to encapsulate the ply. No sense in expending cash and manhours to gain material properties that you don't need.

[Venchka]

Can you get exterior fir Medium Density Overlay up there? The finish on MDO is hard, smooth, tough, stops checking and grain print through and paints nicely. This is the plywood exterior sign makers use. It may also be less expensive than marine plywood.

[DougWilde]

You gain ding resistance when you add the cloth, fiberglass or Dynel, etc.

Do a simple experiment. Prepare three samples of the plywood you are going to use.
On the first apply the finishing system (just the paints, no epoxy) you intend to use on your boat.
On the second put on the epoxy, then finish with paint.
On the third start with the cloth and finish as you intend.

Now drop a screwdriver, blade first, from a height of something minor, say one foot, onto each sample. If your experience is similar to mine, the blade will penetrate the painted and epoxy/painted samples but not the cloth covered one.

My conclusion: either just paint or epoxy with cloth. Coating with a thin layer of epoxy on a good grade of plywood does little to protect your boat because breaching the protective (epoxy) layer is easy.

I sheathed my boat and where I didn't it's just wood and paint/varnish.

Doug Wilde

[gunsmith]

MDO is available but fortunatly or not, the plywood (fir) came with the deal. A test is a good idea. Print through is what I would like to avoid. On my verticle panels I may go without the cloth but on my roof and deck areas, glass will be part of the make up.

[JimD]

Especially if its fir, if it were my boat I'd glass it. Either way the surface will still have to be faired if you want it to look nice.

[NormMessinger]

The MDO I've seen, only casually when visiting a sign shop, does not show print through. It is made to stand up to the elements in road side signs and the like. I rather doubt epoxy would penetrate the overlay. Right? I wouldn't expect voids in the interior plies either. 3/4" MDO aughta make pretty good cabin sides if just painted, eh?

[High C]

With the fir plywood you already have, the cloth is a must. 6 ounce should be about right. If you don't use cloth on that fir, it'll look like the back of somebody's old garage within a couple of years. :eek: