The black line where the bath meets the wall: strip it, seal it, and it holds for years

The mouldy join along the bath edge is the most DIY-attacked seal in any house and the most commonly ruined by the obvious shortcut: squeezing new silicone over the old. The old bead is mouldy behind and outgassing its edges, and new on top lifts with it inside months, sealing the black in place for the next coat. The whole correct job is removal, dryness, one bead - it costs an afternoon and holds for years, which is why it is worth doing exactly once.

Strip it properly, and the word is dry

1. Score both edges of the old bead with a sharp blade (a hooked caul-removal tool beats a razor for keeping knuckles out of the tile), pull it out whole along the join, and chase the stubborn ribbons with the blade flat against the tile - no metal edge into the bath's acrylic lip, which marks permanently.
2. The residue nobody sees is the real problem: any film left in the gap is the reason the new bead peels. Solvent on a cloth, then mould remover or diluted bleach on the bare gap for its full dwell time, rinse, and now the part everyone skips - dry it hard: open window, fan, an hour of hairdryer on cool-air distance, then leave it another day of normal bathroom use with the shower curtain closed. Silicone will skin over a damp gap, bond to nothing, and fail exactly where you didn't look.

One bead, tooled, then ignored for two days

Tape both sides of the join for the straightest line the first time - not a rule, but tape is cheaper than re-doing it. A tube of sanitary, mould-resistant silicone (the neutral-cure ones smell less and grab to acrylic better than acetic "vinegar" types - the label says it), cut the nozzle smaller than you think - 4-5 mm, at 45 degrees - and run one continuous squeeze along the join with the nozzle trailing, not the finger dabbing behind. Tool it within ten minutes: a wet fingertip and soapy water in a spray bottle (plain water marks acrylic), one confident swipe along the length, tape off while the skin is still tacky, and then the hardest instruction in home maintenance: nobody touches the shower for a full two days. A cure is a chemical clock, not a drying; early water etches the surface and restarts the mould clock years early. Two small habits after, for free: a squeegee pass down the walls and the bath lip after use, and the extractor or window ten minutes on beyond the steam - the grout-mould chapter is the same moisture story in another language, and the two fixes double each other's life.

When the bead is the symptom, not the disease

Re-caulked join blackening again within months means water getting in behind it - a cracked tile over the tray, a failed vertical corner joint that the tape never reaches, or a bath that flexes when someone steps in, tearing the bead from underneath (press the corner while stepping - if the wall moves with you, the fix starts with the pan supports, not the tube). And any black that spreads into the grout field or softens the board behind the tiles is the slab-water conversation from the leak chapters, not a silicone conversation - the desk's line on all of it: a fresh bead buys a perfect canvas; a leak paints on the back of the tile.