You can easily remove rust in one of two ways: Scrub with a commercial cleaner and an abrasive pad, or power it off using a spinning wire-wheel brush in a drill. Both methods work well, but with key differences you should know about.
Tough rust cleaners
Empire TopSaver, a commercial product, has a solvent-based, penetrating formula that breaks down rust. You simply spray it onto the affected area and scrub with a scouring pad or steel wool. After a minute or two, wipe away the slurry with a clean towel, photo below. If any rust remains, repeat the process. TopSaver's formula also has a sealant that works to prevent rust from returning. Although it removes rust effectively, this product does not get rid of dark stains embedded in the cast iron.
Boeshild Rust Free
Boeshield Rust Free attacks rust with a phosphoric-acid formula, so when applying it wear latex or rubber gloves to prevent skin burns. Spray Rust Free onto affected areas, allow it to penetrate for 30 to 60 seconds, and then scrub with a scouring pad or steel wool, middle photo. (If left on more than a couple of minutes, Rust Free will create dark stains that could become permanent on cast iron.) Repeat if necessary. This product removes the rust as well as dark stains, but it dulls the patina, bottom photo. If you're cleaning only part of the top, this dull look will be noticeable.
Power away rust
If you don't have either commercial cleaner, use a wire wheel in an electric drill to loosen rust particles, photo below. Then wipe the surface clean with acetone. On light rust, this method proves about as quick as the cleaners.
A wire wheel works better than the cleaners on deep rust, though. Keep the spinning brush flat on the surface to avoid scratching the cast iron. If you should create scratches, you can remove them with 400-grit wet/dry sandpaper and a light lubricant such as WD-40. Switch to 800 grit and repeat the process if you still see scratches.
Now, seal the deal
Once you get your cast- iron top clean, apply a sealer, such as TopSaver or Boeshield T-9, to prevent rust from returning. Do it soon after cleaning; wait until the next day, and you could have quickforming flash rust to remove. Apply sealer every 2 to 3 months in humid climates, and every 6 months in drier areas.
TopSaver: 8 oz., $18, Empire Manufacturing, 866-700-5823, empiremfg.com.
Boeshield Rust Free: 8 oz., $11, PMS Products, 800-962-1732, boeshield.com.
Boeshield T-9: 12 oz., $14, PMS Products, 800-962-1732, boeshield.com.