Remove bathtub rust stains using a paste of baking soda and lemon juice applied directly to the stain. Let it sit for 30 minutes then scrub with a soft brush. Commercial rust removers containing oxalic acid work on stubborn stains. Bar Keepers Friend is effective and safe for most tub surfaces. For porcelain tubs pumice stones gently buff away rust. Avoid bleach which can set rust stains permanently. Address the rust source such as corroding pipes or fixtures.