This is a completely mad tip that comes with a government health warning.
I picked this one up from an American guitar site – I mislaid my notes as to which one so cannot give a credit.
Basically, if you have brittle fingernails – my middle and ring finger nails grow with a “fault” which can split open – then this is a potential solution.
If you are foolhardy enough, then go to your local equestrian supply stores or farmers co-op and buy a tub of animal hoof care lotion. I bought one called Cut Heal Hoof Heal 5 in 1 Hoof Care for Horses, Cattle and Goats. After applying this to my fingernails just a few times a week, I found after about 4 weeks that my nails were flexible but stronger and that the troublesome splitting was no longer giving me the same problem.
The tub of lotion is so large (454ml) that I thought about rebottling it in 5ml bottles and selling these as Guitarist’s Nail Care Lotion for £10 a time. However, I thought I might end up being sued by professional players whose finger nails had dropped off.
So, follow this tip entirely at your own risk and only after consulting a qualified medical practitioner.