Metal tool box

Hi Kendra. Thanks for your guidance. I inherited an old metal tool box that I want to spruce up with a great new coat of paint and some new hardware.  However, there is existing rust on it. I plan to sand off the rust, but am not sure if I should also sand off the old paint. And, if I should use a primer then to seal it as I may end up painting it a lighter color than the original paint.  I know that some of the Rust-Oleum spray paint that I have are a combination of primer and paint. Can you help me to understand the difference? When should I use  a combination spray paint/primer, and when should I use a straight primer and then come back with a coat of spray paint?  Thanks. Elizabeth