The pilot reported that he landed the tailwheel equipped airplane on the main wheels and upon slowing to about 20 mph, lowered the tailwheel to the runway. The airplane veered to the right, and the pilot attempted to control the direction by applying the left brake. He “overcorrected” to the left, and the airplane ground looped and veered off the left side of the runway. The inspection of the airplane revealed substantial damage to the right wing spar. The pilot stated that he should have maintained directional control by adding power instead of applying the left brake. He reported that there was no mechanical malfunction or failure of the airplane prior to the ground loop.