Abstract: A method of protecting a plastic boat hull against blistering, comprising the steps of applying an outer gelcoat layer to the inner surface of a mold, applying a layer of barrier coat material of microspheres thoroughly mixed in a synthetic resin matrix to the outer gelcoat layer to form a barrier coat layer, bonding the barrier coat layer to the outer gelcoat layer, applying an outer layer of fiber reinforced synthetic plastic to the barrier coat layer, bonding the outer fiber reinforced synthetic plastic layer to the barrier coat layer, applying successive layers of fiber reinforced synthetic plastic to form a laminated boat hull having a series of fiber reinforced synthetic plastic layers with an inner layer, and applying an inner gelcoat layer to the inner layer of the fiber-reinforced synthetic plastic layers.