Capt. Tolley's Creeping Crack Cure Hairline Crack Sealer. (I bought it just because of the name).
At $8, it's a quick, easy, cheap and clean fix, and can be used on any
dried surface or any applicaton, inside or out, fiberglass or aluminum, boat or RV.
It's available from
West Marine catalog sales—among
others—in 8-oz. or 2-oz. applicators. One source suggested using the 2-oz applicator, as it's got a smaller nozzle, and refill it with the 8-oz. ($13).
I've used this product on fiberglass cracks, though it will seal the smallest of leaks in most anything. According to the manufacturer, it is a copolymer, so it is thinner than water, and "wicks" into the smallest of cracks using capillary action. You can apply other sealers over it.