I've been rolling several different kayaks with a single layer (inside and out) of plain-weave 4oz cloth for about 5 years now. No problem, no signs of stress, no idea white spots.

> In rolling the hull is stressed where your knees push on the
> deck. The flexing of the deck in this area causes the hull to flex.
> The flexing causes the fiberglass to buckle and then to fail at the
> glue joints in wood strip boats. Rows fo white spots are the first
> indicators.