Ive done quite a few shells using spray paints from motorfactors and tamiya paint. Car paint is fine to use but when you hit a kerb it chips really easy, i would recommend using tamiya paint as it can take abit more abuse. Just spray light coats until your happy that it is painted evenly and allow it to dry for abit between each coat as the paint can run very easily (i noticed the paint runs more easily when using motorfactor paint, tamiya paint will run but not as easily) hope this helps you.
Makesure the shell is cut, drilled etc and smooth before spraying, and dont remove the overspray film on the outside of shell until your happy with the way the shell looks then add your decals at the end.