Removing silicone bathtub caulk is an involved process. All traces of old silicone caulk must be completely removed. New silicone caulk will not adhere to any residue of the old caulk. There are products that are made for silicone caulk removal, but it will still require much scraping with a caulk removing tool. If the average homeowner wants to attempt this job, be aware it will take several
. hours. More tips and information can be found here http://www.houserepairtalk.com/f33/bathtub-caulk-5981/.