prime or undercoat paint on banisters

Ayah use satin finish, gloss is old fashioned and no wonder it's sticky, must put on very thin as in microns

Ideally you should prime, undercoat and then put the top coats on. If you want a good looking and durable finish that's the way to go. Water based paint is healthier and just as tough as long as you allow it time to cure properly (sometimes a couple of weeks). If you really want a high gloss finish with water based paint it's not easy and you need at least two top coats and sand it down in between coats with very fine abrasive. Remember - the primer gives you the adhesion you need (to stop paint flaking). The undercoat gives you a good base and the top coats give you the protection.