It's also quite possible that the white stuff is down deep in the laminate, rather than a surface condition. When you bruise or stretch fiberglass or other laminated fabrics, they fracture slightly, down inside the weave. When you fracture the fibers, they turn whitish. A fractured area of woven cloth will usually show the weave pattern. They may not have any serious structural effect, but there is no way to get rid of them aside from removal and replacement of the damaged area. You can polish and varnish the surface all you want, but unless you're sanding way down into the cloth (which would weaken it greatly) you're not getting to the source of the problem and the spots will still look about the same.
The main causes for this type of thing on wooden canoes are impacts and heat. The wooden hull is much more prone to expansion and contraction from climate conditions than the laminated skin is capable of matching. Figure that if your 15' canoe expands as little as one half percent in length due to heat or weather conditions, that's almost 1" of change. Fiberglass and most other similar composites can't stretch that much over 15' without damage, and can't recover from it later. Same thing with impacts. The glass may not break, but a good impact might do some fracturing if it exceeds the limited modulus of elasticity of the composite skin.
If the spots are fractures, there realy isn't an effective cure, other than replacing the skin or painting the canoe. Best bet may be to simply live with them and enjoy your canoe. Sanding deep enough to remove them (if you even could) would not be a good idea.
