I studied with a teacher for 6 months in 1976 at U. Of Idaho who had been a student of Segovia's for 5 or 6 years in Spain. I moved and could not continue lessons and I wish I remembered his name.
1st lesson was all about the nails. He looked at my hands and said, " oh these will never do"
My history at that time was electric and steel acoustic.
So he studied my hands and set forth the following that if I wanted to continue I would follow and he meant it and I followed .
Left fretting hand fingers trimmed low, well below, really as low as could be trimmed. This of course to fret proper and not place divits in the fret board. String fingers, your file must lay flat on the top of finger, notice how the file sits, you want a slight angle up so the finger nail just tops the bulb of the finger tip. To use the nail if needed, it is a support. You must carry clippers and file with you at all times if you wish to keep your nails.
20 minutes of my hands being carefully manicured by this tall man with longish hair and beard to match wearing flannel and jeans. He stated I must find in my diet items that would strengthen my nails, he suggested a glass a day of whole milk and good cheese from whole milk twice daily with meal or snack. He suggested I wear gloves until my nails got thicker if I was outside doing labor. So noticed in two weeks I had nails to file and shape void of cracks and chips. At the end of first month, perfect nails filed and shaped according to him. I'm still working on proper technique etc. when I forget to do what I was taught I end up with cracks and chips and go through my monthly files, milk and cheese. Ok so yes I'm a new guy, I'm 61 and you guessed it I talk a lot. I've tried calcium supplements as well which would be easy in comparison. Well we all know, nothing is easy.