Naum Shtarkman (piano), Lou Lim Ioc Gardens. November 5 NEITHER floods, nor mosquitoes nor an inferior Kawai piano could hide the ultimate surprise of the Macau Music Festival last Friday. Ukrainian-born Naum Shtarkman, winner of the 1955 Chopin International Competition, rarely ventures out of his country. Those who came to hear him play with the Hong Kong Philharmonic last Thursday were denied a special visit (The Forum was flooded out). But at his recital in Lou Lim Ioc Gardens, Shtarkman proved to be one of the great secrets of the piano world. The Gardens are not the ideal venue for a pianist of such sensitivity and muscularity. In his delicate ''Moonlight'' sonata, the sounds appeared tinny, the pauses seemed contrived. But put him into more muscular works, and Shtarkman's mastery became apparent. Even in the most powerful passages of Schumann's Carnaval, Shtarkman allowed the transparency of those counter-tunes to come out. He could be as graceful as was needed (almost ironic next to his impassive bulldog face), and powerful enough to make the climax a thrilling one. The Tchaikovsky Theme and Variations is rarely played, and Shtarkman gave it a full-bodied reading. But it was the final Chopin which showed him at his romantic best. The second Ballade, second Scherzo and the Opus 66 Fantasia showed a composer equipped with the strongest physical gifts, holding back and presenting a palette of fragile hues and charming songs.