A method to form a sealed, partially or fully densified fiber reinforced composite preform which comprises: impregnating said preform with a major portion of a liquid, ceramic-forming precursor; initiating cure of the outer surface of said precursor with radiation, such as actinic radiation; a thereby forming a sealing barrier or skin on the surface of said preform. In a second embodiment the preform is infiltrated with a thermoplastic as liquid, frozen, excess frozen infiltrate optionally removed from the preform surface, then the preform is coated with a liquid which is curable with radiation, cure is initiated with radiation and the preform is sealed with a barrier coat.