OPI Nail Envy (original formula and Matte formula) works but you need to be religious about applying two coats daily.
Sometimes nails get brittle and start to peel. My parents are nurses and they worked with geriactric patients that have the yellow, peeling, crusting, brittle nails that we all know about. The secret to fighting it is ....
Jello.
I kid you not. Plain jello. Football players in the 1970s used to swear by it because it prevented their nails from getting weak.
I can attest that it was the only thing that kept my grandmother (and great-grandmother's) nails strong for years. She had it with dessert nearly every day, approximately 4 spoonfuls. Mamaw was a diabetic so she struggled with keeping her nutrition and sugar levels in check; sugar-free worked fine for her. When she was in the hospital, just before she died, the dieticians removed Jello from her diet. Her nails became very brittle and weak; they snapped and peeled with the slightest pressure. My dad got them to put Jello back on her diet, and her nails eventually became as they always were - strong, white and beautiful.
Other things should be noted though. She was in hospital, so she didn't have her fingers in water all the time. That helps too. She didn't receive must moisturisation on her hands though, but that helps me.