Is it ok to spray this on already rusted metal with no prep work done?

Happy to clear things up! Proper surface preparation is essential for a successful application with Rust-Oleum Automotive Undercoating Textured Enamel Spray Paint. Surfaces must be free of dirt, dust, oil, grease and any contamination that may interfere with adhesion. Remove any loose paint and rust with a wire brush or sandpaper. Glossy surfaces should be lightly sanded to create a surface profile. Mask off area to avoid overspray on the auto finish.

This surface should not be rusted.