Cleaning Mildew on Caulk

A collection of cleaning supplies in primary colors.
What You'll Need
Scraping tool (such as a utility knife)
Silicone caulk
Lacquer thinner
Heavy work gloves
90 percent isopropyl alcohol
Alkaline chemical
Tri-sodium phosphatE
Powdered laundry detergent

Q. I moved into a new construction three years ago. I have a problem in my bathroom. The white caulking lining at the bottom of my walk-in shower, between the ceramic tiles and the shower base, is beginning to show small gray/black spots. I think this may be mildew forming under the caulk. I tried to clean the spots with the usual household cleaners, but to no avail. Short of replacing the caulk, is there any other cleaning chemical that can be used that will penetrate below the caulk surface to remove these spots?

A. If the stain is below the caulk, don't waste your time buying all sorts of cleaners; you will need to remove it and replace it.

Removing Caulk

The difficulty in removing the old caulk depends on the type of caulk that was used. No matter what type of caulk it is (non-acrylic water-based, acrylic water-based, or silicone), first scrape out as much of the old caulk as possible. Don't try to remove it all by scraping or you will scratch the surfaces.

Wear heavy work gloves and plan on taking your time for this job. Use a utility knife or a strong scraper. Razor blades can be used, but care must be taken to not scratch the surface, especially with fiberglass and plastic surfaces. If you try to rush, you will either end up cutting yourself or scratching the tile, bathtub, or plumbing fixtures.

If the old caulk is non-acrylic, laying a wet rag on it for several days will make the material swell and allow you to peel off most of it. If it is acrylic caulk, 90 percent isopropyl alcohol should make it lift off the tub and tile. Silicone caulk is the most difficult to remove because it is resistant to most solvents. Some strong alkaline chemicals might weaken the old silicone. Follow the manufacturer's instructions carefully. Ensure that solvents are not harmful to fiberglass products.

Inhibiting Growth

Mildew thrives in most bathrooms because it is the perfect breeding ground for it. There are some things that you can do to retard its growth.

  1. When you are done bathing, rinse the walls and caulk with water to minimize soap and oil residues. Sure this takes time, but what is more important to you?
  2. Leave the shower doors or curtain open after bathing so that it dries faster.
  3. Run the bathroom vent fan for about five minutes. If you don't have a fan, get one installed, regardless if the room has a window. Hardly anyone opens the window!
  4. Paint the walls with a glossier paint. Flat wall paints attract mildew more readily than shinier paints. Add a mildewcide to the paint before painting

Applying New Caulk

Cut the tip of the tube of caulk at an angle and size the opening to the width of area you want to fill. For acrylic water-based caulk, you will need a bucket of clean water to clean and smooth the caulk bead. For silicone, you will need lacquer thinner. Lacquer thinner is flammable, so be weary of sparks and open flames, and open a window. Ensure that it is not harmful to fiberglass products.

Apply the caulk slowly and only about two feet at a time. You can do one side at a time but work fast as this will start to form a "skin" soon after application. Use your finger to smooth the bead. Wipe the excess from the tile and tub with a wet sponge (water-based caulks) or a rag with lacquer thinner (silicone caulks).

There are new bathtub caulks that are truly mildew-proof with lifetime guarantees. All bathtub caulks have some mildewcides in them, but the more expensive caulks have much more of these chemicals. Every time water touches the caulk, a bit of these chemicals leach out to block mildew growth. A caulk with a higher concentration of these chemicals stays mildew-free for a longer time. Check the label on the tube of caulk for the word mildew-proof, not just resistant.

If you use a silicone caulk, do not use a "reduced odor" one. Lots of silicones say they are 100% silicone, but the reduced odor types use a different curing system, which makes them yellow over time, and they do not stick as well. Get the stinky stuff, which uses acedic acid during the cure (smells like vinegar). This stuff does not yellow, cannot feed mold or mildew, and sticks like crazy so it will not peel off when you are washing your shower. If you put it on right, you do not need to worry about ever removing it.

To Clean Mildew Ridden Areas

For a gallon mixture use three quarts of hot water and one quart of bleach. Next, add one cup of tri-sodium phosphate. Then add 1/4 cup of any powdered laundry detergent. This will not only clean the walls, but will kill any mildew or its roots.

Your bathroom caulking will now be mildew free! Don't forget to inhibit the growth to avoid having to remove the caulking in the future.

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