To restore the beauty of your tile bathtub or shower unit, replace the old caulking sealant at the top of the unit where it meets the tile. This is a normal part of home maintenance and usually needs to be done annually, if not more often.
To defer this maintenance is to invite water infiltration behind the tile, which will damage the wall board behind it. Old mildew damaged caulk can no longer be relied upon. Its sealing properties have been compromised by the presence of mildew.
HOW TO REMOVE CAULK
To remove caulk, cut along bead with a utility knife. make two cuts -one as flat as possible along the surface of the tub and another held at about a 45 degree angle. The objective is to try and remove the old caulk all in one piece, to the extent possible. It is unlikely that it will all come out in one piece. There are tools available that are generally a one-time use variety that work fairly well for removing caulk. Use a straight razor blade to remove any residual caulk from the surface of the tub and tile.
To get rid of any remaining mildew, add about 6 oz. of bleach to a gallon of water and with an old toothbrush, scrub out the joint with the bleach solution.
Use a new sealant specifically designed for kitchens and baths. You can go with a straight silicone caulk -either clear, white or a color -or, there is a product by Hercules called ‘Plumber’s Caulk’ that I have had success with in terms of how long it lasts. It is advertised as ‘siliconized.’ I find it easier to work with than straight silicone.
I always clean the surfaces to be caulked with rubbing alcohol and a cotton ball or rag. I want the surface to be as clean as reasonably achievable so the caulking will adhere to both surfaces. I do not like doing things twice if I can avoid it. If the surfaces are not adequately cleaned and the caulk does not adhere properly, it then falls in the category of re-work. There is nothing worse in my view.
Place a bead of caulk into the joint -no larger a bead than is necessary. Trim the nozzle to the appropriate size. There are different techniques to finishing the bead of caulk. One is to use a soapy water solution to smooth the bead with your wet finger. Another is to apply the bead in front of the nozzle -effectively pushing the nozzle along while squeezing the caulk out in front of the nozzle. With practice, one can lay down a nice bead that doesn’t require your finger.