To get it smooth enough to paint, you will have to sand, sand, and then sand some more. What I usually do after I get the glass sanded, is put bondo or gel coat over the sanded glass and that makes it glass smooth. Then you can primer and paint it, but make sure you use filler primer.
It is a long and time consuming process, but in the end it is very worth it.
Just be prepared to sand, wet sand, sand, wet sand, etc.... etc....