This is a job I get asked to do all the time. It doesn't matter what you use or how much you clean, eventually the silicon around your bath or shower or sink or worktop will degrade, come off, or get mould on it. The process involves scraping the bulk off, then user a silicon remover substance. I then scrape the rest off and clean and dry the area thoroughly. After this I can apply the new silicon and leave to dry. If you need it doing get In touch.