How do you repaint a steel security door?

You'll need to remove all rust, either sanding it off or use a sandblaster to remove all of the rust. Then, when all of the rusted metal is sanded or sandblasted clean, use 220 wet/dry sandpaper on any remaining paint to remove any glazing of the old paint. DO NOT ALLOW ANY LOOSE PAINT TO STAY ON THE METAL. Then, use an "etching primer" to stabilize any rust that you may have missed. Even if you get it pretty clean and can't see any rust, there will be some left behind that you will miss so the etching primer is pretty helpful. After it's primed, sand it smooth, making sure that you don't sand through to bare metal. Then spray it with a good quality oil based enamel or catalyzed paint, possibly even a high quality automotive grade paint.