I learned in school that “grip” involved fingers and (opposable) thumb; never thought about how one finger by itself has a kind of grip on a key—until 21 years ago, when I lost some sensation in my right hand. This makes me work harder; in big lateral shifts, I use a different trajectory so as not to go sailing off the key I land on. I come at the key more vertically, which takes a moment longer.
I could fix the problem if I found a material for keys or fingertips with just the right amount of stickiness. I’ve considered-—
Water: a few drops on fingertips work great, but last only a moment.
QDA (“Quick-Drying Adherent,” rosin in aerosol form, used by athletic trainers so skin won’t chafe when wrapped with bandages): works well on fingertips, but wears off before intermission; and takes longer than intermission to dry, so you do without it in the second half.
Aerosol hair spray, as used by Arthur Rubinstein (I’m not the only pianist with the problem): psshhht psshhht up and down the keyboard; but the sticky layer must be cleaned off after the performance. Also, I’m nervous about getting it in the gaps between keys, maybe gunking up the mechanism.
3M prototype materials intended for fingertips of aging surgeons (pianists are not the only ones with the problem): didn’t work for me, but thank you, 3M Medical Products Division.
Wax, rubbed from candles to make small, thin patches on keys, as a European lady pianist does: Works perfectly, and it lasts; but collects dirt like crazy; tedious to clean off. And if other people use the same piano, they will want you to clean daily.
Last week I found the best answer: the problem spontaneously vanished. Pretty striking, after 21 years! But it happened previously when it started in 1978 and vanished in 1980. In that earlier occurrence, symptoms were identical but not so strong: I didn’t have to change how I played.
I wonder how long this new remission will last! 
