Black mould in the silicone does not clean off once it is in the bead. It has to come out and go back in. Done properly it stays clean for years.
No, and anyone who offers to is not doing you a favour. New silicone will not bond to cured silicone, so it lifts away within a few months and traps water behind it. The old bead has to come out.
Nearly always ventilation. If a bathroom cannot dry out between uses the mould returns no matter how good the silicone is. Worth checking the extractor fan actually pulls, and that the bath is not flexing when you stand in it.
Give it 24 hours before it gets wet. It will be touch dry much sooner, but it needs the full cure to seal properly.
Covering Worthing, Shoreham-by-Sea, Lancing, Portslade, Hove and Brighton and the surrounding Sussex coast. Based in Worthing, fully insured through Markel.
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