I'm an adult who has played and quit off and on throughout his life. I have two main physical problems with my hands. The one I'm writing about here is that when I use my thumbs, particularly on the black keys, it's largely the nail that touches the keys, with the result that the thumb slides and pivoting becomes precarious. I can't change this without upsetting the normal position of my hand and other fingers. I sometimes use a finger from a thin kitchen glove over my r.h. thumb. But I'm wondering if anyone else has dealt with this problem and perhaps even solved it.