Do you need to use a primer on rusty surfaces first?

It is not necessary, but using a primer before applying Rust-Oleum Satin Spray Paint will provide even longer-lasting protection. For extremely rusty surfaces, use Rust-Oleum Rusty Metal Primer. http://www.lowes.com/pd_99021-90-7769830___?productId=3729855&pl=1&Ntt=stops+rust+primer+spray

You will need to sand down the rusty surface to get it as smooth as possible. If there is a large area of rust, after it is prepared, it should be primed.

If long-term protection from further rusting is needed, then the answer is definitely yes. The only exception that I can think of is where a rust converter is used to chemically turn the rusted surface into a protective layer, but this is effectively equal in scope-of-work to priming. In either case, a good wire brushing should be done on heavily rusted areas (an abrasive disk or sander may be OK where only light surface rust is present). I know it seems like a major PIA to carefully prep and prime first, but it really does save a lot of time, labor and money in the long run! Good luck.