Any nail hardener is just going to temporarily fix your problem. But eventually it stops to work.

#1. Polish (even clear coats) deprive the nail of oxygen, which causes serious damage to the nail
#2. Polish remover is even more damaging
#3. Think of brittle nails like this. You have a piece of rotting wood and you want it to look good so you paint over it. Realistically all you did was hide the problem. You need to cure the problem!
#4. Nail hardeners are thick polished enriched with things like calcium to make them sound good, but they are just a polish.

If you want to rebuild your nails you need biotin. There are tons of biotin supplements out there. Some good and some bad. ELON Matrix Plus includes l-cycteine and silica to help the nail growth as well. And if you have any thinning hair problem take the ELON Matrix 5000. You get stronger nails and hair in just 6 months!