All My Garden Spade products are made to last and built to the highest specification. You can leave them in the garden, but why would you?
They should become your friends, they slot into your hands and are nice to hold. They are not intended to be used as hammers, mallets or for making cement. This will shorten their life and blunt the blades.
Simply wash off any earth from the blade and once in a while, apply with a cloth some teak oil to the wooden handle. Over time the handle will smooth even further with age. The paint will eventually wear away, this is normal and is only there for cosmetic effect.