For the safety and convenience of personnel working on upper portions of a blowout preventer stack, a work platform is supported at the midheight of the stack. The former can readily undergo one-electron oxidation to give radical cations, whereas the latter are easily reduced to afford radical anions. The method includes first smoothing the HTS surface by Gas Cluster Ion Beam bombardment, followed by annealing in partial pressure of oxygen to regrow the damaged surface layer.