I'm a lefty too, but learned how to airbrush right handed... weird huh? Guess it goes back to learning calligraphy... it was easier to learn right handed, since I smudged everything trying to do it left handed... okay, way off topic now... Anyways, I notice if I go a few days without painting, my whole hand gets tired much faster than when I'm painting every day. I usually just shake them out and it usually works itself out, but I never thought about using my thumb... might feel kind of strange at this point.