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Cauking can contain mildewcides and fungicides. The only problem is that these chemicals can be water soluble, meaning the active ingrediants that prevent the mold growth can wash away after time.
Caulking may appear smooth with the naked eye, but it is somewhat porous and thats where the mildew and mold spores hide. Another problem is that many soaps and body washes contain ingrediants that mildew loves to feed off, so your soap is actually making the caulk a perfect place to thrive.
Caulking brands that mildew quickly only contain a small trace amount of mildewcides, while the anti-mildew types contain a great deal more. I guess the intention is that more is better and it will take a lot longer to wash away, the fact of the matter is you will need to re-caulk anyway but the anti-mildew brands will last longer.
Getting your Caulking to Stick properly:
Caulking ingrediants are basically an adhesive. In order for it to adhere properly your joint must be dry and free of dust, but you did read the caulking instructions? didnt you?
When I caulk anything like a bathtub or my countertop, I always read the label and make sure the product is 100% silacone. Why 100% silacone you ask? Because it simply lasts longer and sticks better. I don't like the new latex caulking as its harder to work with and it will degrade much faster. I have tiled hundreds of bathtub surrounds and always use the real silacone.
I dont use any of those fancy products you can purchase for removing caulk, the best thing to use is a razor scraper (wallpaper type remover with the replaceable 4 inch blade, Richards makes the best ones, look for the yellow handle.) or possibly a utility knife just to loosen it up a bit first.
Preventing Mildew in Baths
Sounds simple, but run the fan for 20 minutes after your shower. If your bathroom has a window, it dont hurt to open it. Mildew grows faster in a wet environment, so using a sqeegee to remove most of the water is good too.