How to re-caulk shower, tub with mildew

The bathrooms have no windows, but there are fans. When I recaulk I make sure the shower or tub hasn't been used in a few days so the area is good and dry, and after caulking, I wait two or three days to be sure the product is completely set.

Please, provide some advice or perhaps help me find a product that works.

A: The caulk is failing because of residual moisture in the air after you bathe or shower. It's good that you have fans in both bathrooms; in fact, they are required by code. But fans usually aren't enough. We believe fans, especially less efficient ones found in older homes, don't move enough air out of the bathroom to remove relative humidity.

It's unfortunate that you don't have windows. Opening a window and leaving the bathroom door open would provide cross ventilation and quickly rid the room of excess moisture.

As we've said many times before, mold needs three things to thrive: air, moisture and a food source. You've got all three. The ambient air is moist from the bathing and showering, and the caulk is the food.

Here's what we'd do to reduce the time you spend operating a caulking gun.

Leave the fan on for 10 minutes or so after each shower. Leave the bathroom door open as much as possible between baths and showers. Wipe down shower after each use to limit dampness.

The next time recaulking is needed, your first job is to remove all the old caulking. A teardrop-shaped paint scraper or the tip of a flat head screwdriver works well for this task. Removal of all the old caulk is particularly important if there is mold or mildew present.

With the old caulk removed, give the area a good rinse with chlorine bleach to kill the mold spores that remain. Use the bleach at full strength and wear rubber gloves for this step. After letting the bleach work for 10 or 15 minutes give the area a rinse with a damp cloth to remove the bleach residue. Then dry the area with a cloth.

Wait a couple of hours to make sure the area is dry. Or better yet dry it with a hair dryer. In any event, the surface must be completely dry before re-caulking. For a visual of this process, go to www.wikihow.com/remove-mold-from-caulk.

The final step is to recaulk the joints. There are a number of products that contain a mildewcide. Because we don't know which you've already tried, we hesitate to make a solid recommendation. We suggest you do a Web search for "mildew-resistant caulk."

There's no magic bullet here. The best you can do to avoid living with a caulking gun in your hand is to practice good moisture control by drying the tub and shower after each use, leaving the fan on longer and keeping the bathroom door open as much as possible.

The Burnett brothers are freelance writers. E-mail: sweatequity@sfchronicle.com