Bathtub caulking seals the joints where the tub meets the tub surround to keep water from dripping down along the walls and under the tub. The tub caulking is constantly exposed to moisture and can sometimes become covered in mildew or mold, which appears as black spots along your caulk. Below are steps that will remove mildew from tub caulking using toilet bowl cleaner containing bleach.
Squeeze the toilet bowl cleaner with bleach out in a thin line, just enough to cover the caulk where mildew is present.
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Let the toilet bowl cleaner liquid sit on the mildew-stained caulk for 30 minutes or more, depending on the severity of the stain. The toilet bowl cleaner clings to the caulk better than liquid bleach and does not have to remain on the mildew as long to remove the stain.
Check on the mildewed caulk after 30 minutes--you will be able to see through the toilet cleaner liquid to determine if you need to keep the liquid on for more time.
Rise the toilet cleaner off of the caulk with water using your showerhead or sprayer attachment. If you cannot reach the caulk with your showerhead, you may have to wipe it off with a wet cloth.
Examine the caulk for mildew stains and repeat the process if necessary.