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That dark line running along the base of the tiles is almost never just surface dirt. Black mould on silicone is usually growing inside the bead and behind it, which is why scrubbing, bleaching and endless bouts of elbow grease never quite win. Silicone is slightly permeable, and a bathroom offers mould everything it needs: warmth, moisture and a steady supply of soap scum to feed on.
Once the growth is established in the silicone itself, replacement is the only real fix. Bleach may lighten the colour for a week or two, but it will not reach the mould behind the bead, and repeated use degrades the sealant and leaves it more prone to lifting.
It is also worth asking why it came back so quickly. If the same joint goes black every few months, look for a failed grout line, a gap where the tiles meet the tray, or a slow leak behind the wall. Sorting out the moisture source is what keeps a new seal looking new.
Avoid metal scrapers on acrylic baths and shower trays. They scratch the surface, and every scratch gives mould a fresh foothold.
Run the knife along both edges of the bead, tilting the blade away from the tray so you are cutting the silicone rather than digging into the plastic. Work in short sections of 150mm or so and pull the strip away as you go. Longer horizontal joints often come off in one satisfying length.
For the residue left behind, apply a silicone remover and give it the full dwell time on the label before scraping. Anything left on the surface will stop the new sealant bonding, and silicone will not stick to old silicone. Once the joint is clean, wipe it with methylated spirits and let it flash off. Skip white spirit or turps at this stage: they leave an oily film that ruins adhesion.
With the old bead gone, you can finally reach the mould itself. Apply a fungicidal wash along the joint and, if the grout is open or crumbling, work it into the gap. Let it sit for the contact time stated on the bottle, then rinse thoroughly and wipe dry.
Now comes the part most people rush, and it is the single biggest reason new seals fail. Silicone will not bond to a damp surface. Warm the joint gently with a hairdryer on a low setting, then leave it to dry completely — overnight with the extractor fan running is ideal, particularly in a windowless bathroom.
While the area is open, check the grout. If it is soft, cracked or missing, rake it out and regrout before you seal. Sealing over damp, failing grout simply traps moisture and builds a mould sandwich behind a brand-new bead.
Run masking tape down both sides of the joint, leaving a gap that matches the bead you want — usually 6–8mm for a bath or tray edge. Press the tape down firmly so the sealant cannot creep underneath.
Cut the nozzle at roughly 45 degrees to suit the joint width, load the cartridge and gun the sealant in at a steady angle, pushing rather than pulling so the bead is forced into the corner. Run each length in one continuous pass; stopping and starting leaves lumps that no smoothing tool can hide.
Smooth the bead in a single movement with a profiling tool, or a finger dipped in soapy water. Then peel the tape away immediately, while the silicone is still wet, pulling it back at an angle away from the joint. Do not touch the finished bead afterwards.
Most sanitary silicones skin over within half an hour and cure fully in 24 hours. Leave the shower or bath unused for at least a day, and longer if the joint is deep or the room is cold. Acetoxy sealants smell faintly of vinegar as they cure; that is normal, but check compatibility before using them against natural stone.
Do those small things and a well-applied sanitary seal should give you several years of clean, watertight service — rather than another black line by Christmas.
SiteSeal shares practical, down-to-earth guidance on uk property damp-proofing, waterproofing and sealant advice for readers across the UK.
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