Rusted clawfoot tub - how to remove rust?

Wipe over with WD-40 the paint with a clear metal sealer.

https://www.hunker.com/13424117/how-to-clean-stains-in-an-antique-clawfoot-bathtub

Use a palm sander to remove the rust. Start with a 80 Grit down to 200 grit. Prime and paint. No real way to preserve and prevent the rust. Rust starts inside out. If you've ever seem how a car body rusts out. WD40 leaves a film so paint or sealer won't hold. You can try a marine grade polyurethane but may not last.

buy "naval jelly" at marine store.

Sand off the rust scales. Spray paint the parts where you don’t want the rust to show with Kilz or Rustoleum for outdoors. Use a big screwdriver or tire iron, and a heavy hammer, to poke holes in the bottom of the tub, about ten holes. Fill the bottom third if the bathtub with charcoal briquettes, empty plastic soda and water bottles, big and medium rocks, or something else inert to help with drainage. Add about another third of a bathtub of garden soil from a garden center, then about a third a bathtub full with potting soil. The dirts will settle enough around the drainage layer that you will be left with about 1/4 of the tub depth to plant in. As the tub continues to rust, a mound of dirt will continue to grow through the tub and up from the ground. Don't move the tub, but pull out any inert material that works it’s way out. A five to ten year process. Enjoy! ☺️