Help with painting bathroom fixtures

Hi everyone, I am new to the site and hope you can help. I wanted to paint my brass bathroom fixtures to satin nickel. I started with the bath tub. I sanded the spout and handles very well with 220 grit sand paper. I then taped very well and applied a metal primer. I applied several coats of the primer and waited 24 hours. This morning, I sprayed with satin nickel. Waited several hours only to find that I can rub everything off with my fingernail. I used Rustoleum products. What did I do wrong? It looks great until, but clearly is not functional. Thank you so much for any help, Kay