Most people have a shower over their bath so an effective seal between the bath and the wall is essential. Why silicone turns mouldy and stops working can be down to several factors but one thing will common. Water has worked its way behind the seal.Water will run straight off silicone under gravity. But if water gets trapped behind silicone then it forms a perfect breeding ground for mould to develop. If you can see mould appearing in yor silicone then it has grown through from behind.
This can only mean one thing - you silicone has lost its bond and has peeled away.
There are several factors that can cause this to happen.
Bath Movement
Baths are made from materials that flex slightly. Even though this flex is minimal it can be enough to pull the silicone away from the wall or bath surface. If you eliminate the bath movement you will prevent the same thing happening again in the future.
Faulty Grout
This is another major cause. Grout is slightly porous. Badly applied grout can be very porous. This allows moisture to get behind the silicone where it is in contact with faulty grout. You will notice that the mould tends to start at the bottom of each grout line. Rake out the grout, re-apply the correct formulation and seal the grout to ensure no moisture will penetrate it.
Poor Adhesion
If the walls or the bath surface are dirty, dusty or greasy then the silicone will suffer from poor adhesion. It can peel away even if the other two issues above are not present (but these will certainly exacerbate the situation)
Cheap, poor quality silicone will also have less bonding ability so do not scrimp when you buy your sealant. Buy the best available - you should only need 1 tube.
