Aerohead, other readers,
I'd like to share some of my expertise as a boat builder in payback for the
aero knowledge you share with us every day. (BTW, many thanks.)
There is no way to permanently stabilize the surface of fir plywood. No
matter what you put over it, even a layer of epoxy impregnated fiberglass,
in time it will "surface check." That means the face ply will crack and pull
apart, up to 1/8 inch. It will tear the fiberglass. You can put on multiple
layers of 'glass, but the cost and weight get prohibitive.
A better choice for a wood-only construction would be ply made from okume
or sepele, these are standard marine construction materials. The are quite a
bit more expensive, and they do surface check, but the checks look like
razor blade cuts. The are easily filled-in/repaired during a second paint job.
One nice feature of these plywoods is you can sometimes get it in 5 ft by 10
ft sheets. The cost can be surprisingly high, but a standard sheet is 32 sq ft,
and these are 50 sq ft, 56% greater area.
For a truck cap, I would recommed HDO Overlaid fir ply -- High Density
Overlaid. It has one or both faces covered with a layer of resin impregnated
film, heavy kraft paper I think. The key here is the layer is fully water tight,
and the face ply doesn't check. It takes paint well, no sanding, and the grain
in the ply is pretty much subdued.
A heavier duty version of this is Signal Overlay. It's the stuff the big
overhead freway signs are made of. It is fir ply with a hardwood face ply,
and a HDO on top of that. Excellent paint surface, no grain "show through."