How To Replace Old Silicone

How do you replace old silicone in a bathroom, and which Selleys sealant should you use?

Replacing a perished silicone bead is a beginner-level job that takes about an hour of working time, plus curing. The sequence that works is always the same: cut the old bead out completely, kill any mould that is left behind, dry the joint, then lay a fresh bead of a silicone suited to the surfaces either side of it. For a standard tiled shower or bath surround, Selleys No Mould is the product the Selleys guide specifies for this job — it is shower-ready an hour after application. Where the joint touches a mirror, natural stone or an engineered stone benchtop, use a neutral cure silicone such as Selleys Wet Area Silicone instead.

The single most common mistake is running a new bead over the old one. It does not work, and the rest of this guide explains why, along with the detail that decides whether the new seal lasts two years or ten.

Why old silicone fails in the first place

Bathroom silicone rarely fails because the material wears out. It fails for three reasons, and knowing which one you are looking at changes how much preparation you need to do.

Mould colonisation. Black spotting in a bead is a mould colony that has rooted into the silicone itself, not surface dirt sitting on top of it. This is why scrubbing never restores a mouldy bead — the growth is below the surface, beyond the reach of any cleaner. Soap scum, body oils and shampoo residue are what feed it, which is why the bead at the base of a shower wall almost always goes before the bead at head height.

Loss of adhesion. A bead that has lifted along one edge, or that you can pick at with a fingernail, has failed at the bond line. Usually the joint was not clean or fully dry when it was sealed, or the bead was laid too thin to tolerate the small amount of movement every wet area sees as the building expands and contracts.

Discolouration without mould. Some yellowing is simply age and UV exposure, particularly on beads that catch daylight. It is cosmetic rather than a performance failure, but since the work to replace a discoloured bead is identical to replacing a mouldy one, most people do both at once.

Why you cannot simply seal over the top

This is worth stating plainly, because it is the shortcut almost everyone considers. Selleys' technical advice is explicit: a second bead applied over an existing one may not adhere well to the original silicone. Cured silicone is a poor substrate for fresh silicone — the new bead grips the old one weakly, and the joint fails again quickly, usually along the seam between the two layers.

Where mould is involved there is a second and more important reason. An established mould colony can penetrate a new seal even when that new product contains mould inhibitors. Sealing over live mould traps it against a food source in a permanently damp environment. The inhibitors in the fresh silicone slow surface growth, but they cannot stop a colony pushing through from underneath. Within months the black spotting reappears, and the joint has to be cut out anyway — with twice as much material to remove.

The rule that follows from both points: every trace of the old bead comes out, and any mould gets killed and dried before new sealant goes anywhere near the joint.

What you need

• Selleys No Mould — or a neutral cure silicone such as Wet Area Silicone if the joint touches mirror or stone
• Selleys Sealant Remover
• Selleys Rapid Mould Killer
• A silicone removing tool or blade
• Methylated spirits and a clean cloth
• A spatula for tooling, and a small container of water with a drop of detergent in it
• Mineral turps for cleaning up wet excess
• Safety gloves
• A caulking gun, if you are using a cartridge rather than a squeeze tube

Choosing between acetic and neutral cure silicone

This is the decision that most affects whether the job succeeds, and it is the one most guides skip. Silicone sealants cure by one of two chemistries, and they are not interchangeable across all surfaces.

Acetic cure silicones — Selleys No Mould among them — release acetic acid as they cure. That is the sharp vinegar smell you notice for the first day. They bond well to glazed ceramic tile, glass and porcelain, which is exactly what a standard shower recess is made of, and they are the default choice for a tiled wet area.

Neutral cure silicones — Selleys Wet Area Silicone and Wet Area Speed Seal — cure without releasing acid. Reach for these when the joint touches anything the acid could affect:

• Mirrors. Selleys specifically recommends a neutral cure silicone between the edge of a mirror and surrounding tiles, because neutral cure will not damage the mirror's silvered backing.
• Natural and engineered stone. For a joint between a glass splashback and a Caesarstone benchtop, Selleys recommends a neutral cure product — Wet Area Speed Seal is named as ideal for that pairing.
• Sensitive substrates where the acid released during cure is undesirable.

If you are unsure and the joint involves stone, mirror or an unusual material, neutral cure is the lower-risk choice. On plain glazed tile, either will perform.

Step 1 — Cut the old bead out

Put your gloves on first; a silicone blade is genuinely sharp and you will be working in a confined corner. Run the blade along both edges of the bead to break its bond with the tile and the tray, then lift the length of silicone away in as long a piece as you can manage. Long strips come away far more cleanly than short stabs, so work steadily along the joint rather than picking at it.

Scrape away as much as physically comes off at this stage. What you are aiming for is a joint with no rubbery material left in it — only a thin haze or film of residue that the remover will handle next.

Step 2 — Soften and remove the residue

A thin film of silicone always stays behind, keyed into the texture of the grout and the glaze. It is invisible from a standing position and it is the reason beads fail early, because fresh silicone bonds to that residue rather than to the tile.

Apply Selleys Sealant Remover evenly over the remaining silicone, roughly 5mm high. Leave it to work for at least 10 minutes — that is the removal time in Selleys Sealant Remover's current technical data sheet — then scrape all the softened material off. Heavy residue may need a second application rather than a longer wait. Note that the how-to page on selleys.com.au still says two hours; the data sheet is the later document.

Finish by wiping the joint down with methylated spirits on a clean cloth. This lifts the last of the softened residue along with any body oils or soap film, and it flashes off without leaving anything behind.

If you want more detail on this stage on its own, see how to remove old sealant.

Step 3 — Kill the mould and dry the joint completely

Even with the bead gone, mould spores remain in the grout and the corner of the joint. Spray the area with Selleys Rapid Mould Killer to eradicate residual mould and mildew, then rinse with water.

Then let it dry — properly. This is the step people rush and it is the one that most often costs them the job. Silicone will not bond reliably to a damp surface, and a shower corner holds moisture long after it looks dry. Leave the room ventilated, put a fan on it if you have one, and do not seal until the joint is genuinely dry to the touch. Wiping the area with mineral turps at this point helps remove any residual cleaner, grease or grime.

If the bathroom has a persistent mould problem beyond this one joint, treating the underlying ventilation matters more than the sealant you choose — see how to get rid of bathroom mould.

Step 4 — Apply the new bead

Cut the nozzle at roughly 45 degrees, sizing the opening to the width of the joint — smaller than you think, since you can always widen it but you cannot make it smaller again. Read the instructions on the back of the pack before you start, as application detail varies between products.

Work in one continuous pass along the joint, holding steady pressure and moving at an even speed. The critical part is pushing the sealant into the joint rather than laying it on the surface. A bead that merely sits on top has almost no depth of bond and will peel; one that is pressed into the gap grips both faces and can flex with the small movements the joint sees.

Do the whole run in one go rather than stopping and restarting mid-length. Joins in a bead are weak points and they show.

Step 5 — Tool the bead and clean up

Within 5 to 10 minutes of application, tool the bead with a spatula dipped in water with a little detergent in it. The detergent stops the silicone dragging on the tool, and tooling does two jobs at once: it forces the sealant properly into the joint, and it leaves the smooth concave face that sheds water instead of holding it.

Do not leave this longer than about ten minutes. Once the surface begins to skin, tooling tears it rather than smoothing it, and you cannot recover the finish without cutting the bead out and starting again.

Clean up any excess with mineral turps before the product skins. Cured silicone has to be cut off mechanically, so a minute of cleaning now saves a great deal of scraping later.

Curing — when can you use the shower again?

Skinning and full cure are two different things, and the distinction matters.

Selleys No Mould (acetic cure) skins over sufficiently that the shower can be used 1 hour after application. It still needs 48 hours to cure fully, and through that whole period the bead should not be disturbed by touching.

Selleys Wet Area Silicone (neutral cure) needs 24 hours before exposure to water. By then it has developed a sufficient skin and a watertight seal — again, provided nothing has touched it.

One thing that does not help: a hair dryer. Wet Area Silicone requires atmospheric moisture to cure, so warm dry air has no effect on cure time and can actively hinder it. Ventilation at normal room humidity is what you want.

As a rough guide to depth, silicone-type sealants cure at a rate of about 2mm per day, so total cure time depends on how thick the bead is.

Surface compatibility worth checking before you start

Timber. Wet Area Silicone will adhere to wood provided it is clean and dry. If the timber is bare, apply a coat of primer or undercoat first — raw timber absorbs moisture and moves, and neither helps a seal.

Mirror edges. Neutral cure only, to protect the backing.

Engineered and natural stone. Neutral cure, and check the stone supplier's own guidance for anything unusual.

Acrylic and fibreglass trays. These flex more than tile. Make sure the bead has real depth in the joint so it can accommodate that movement.

Where No Mould should not be used. Its technical data sheet lists clear limits: it is not paintable, so complete any painting seven days before sealing. It is not suitable for aquariums, for contact with food or drinking water, for permanent water immersion, or on metals other than aluminium and stainless steel.

For reference, No Mould's published figures are a skin time of 20 minutes, a cure rate of 2mm per 24 hours, joint movement capability of plus or minus 25% (ISO 9047) and elongation of approximately 400% (ASTM D412 modified).

Keeping the new seal mould-free

The seal you have just laid will outlast the old one by years if the joint is kept clean. Mould needs a food source, and in a bathroom that is soap scum, shampoo residue and body oils rather than the silicone itself. Regularly cleaning those off is the single most effective thing you can do to protect the bead.

Ventilation is the other half. Run the exhaust fan during and after showering, and leave the door open afterwards where you can. A bathroom that dries out between uses will not sustain a colony; one that stays damp will, whatever sealant is in the joint.

A useful habit for storage: leave the nozzle screwed onto the cartridge with a little product squeezed out of the end. It cures into a plug that seals the cartridge, and you pull the plug out next time. Any opened cartridge has a limited life before it cures solid, but this extends it considerably.

Common questions about replacing old silicone

Can I apply a new bead of silicone over the old one?

No. Selleys does not recommend putting another layer of silicone over an existing bead, because it may not adhere well to the original silicone. Cut the previous bead out and reapply a fresh one to the correct depth, well adhered to both edges of the joint.

How soon can I use the shower after resealing?

With Selleys No Mould, 1 hour after application — it will have skinned over sufficiently. With Selleys Wet Area Silicone, 24 hours. In both cases, avoid touching the bead until it has fully cured.

How long does the silicone take to cure completely?

Selleys No Mould needs at least 48 hours to cure fully, even though it is shower-ready in 1 hour. Its technical data sheet gives a skin time of 20 minutes and a cure rate of 2mm per 24 hours. Sealants of this type cure at roughly 2mm per day, so a thicker bead takes proportionally longer.

Can I speed up curing with a hair dryer?

No. Wet Area Silicone needs atmospheric moisture to cure, so a hair dryer will not shorten the cure time and could hinder it. Normal room ventilation is what helps.

Do I really have to remove every bit of the old silicone?

Yes, and this matters most where mould is present. An established colony can penetrate a new seal even if the new product contains mould inhibitors, so any residue left in the joint becomes the starting point for the next failure.

What removes the last film of silicone residue?

Selleys Sealant Remover applied about 5mm thick over the residue and left for at least 10 minutes, then scraped off. Apply a second time if residue remains. Finish with methylated spirits on a clean cloth to lift the last traces along with any oils or soap film.

Which silicone should I use around a mirror?

A neutral cure silicone such as Selleys Wet Area Silicone or Wet Area Speed Seal. Neutral cure products will not damage the mirror's backing.

What about the joint between a glass splashback and a stone benchtop?

Remove all the old sealant, treat the area with Rapid Mould Killer, dry it fully, then wipe with mineral turps to clear residual cleaner and grease. Seal with a neutral cure product — Wet Area Speed Seal suits glass and stone.

Will silicone stick to timber?

Wet Area Silicone adheres to wood as long as it is clean and dry. Prime or undercoat bare timber first.

How long do I have to tool the bead?

Five to ten minutes from application, using a spatula dipped in water with a little detergent. After that the surface starts to skin and tooling will tear rather than smooth it.

How do I clean up excess silicone?

Mineral turps, before the product skins. Once cured, it has to be cut away mechanically.

How do I stop the leftover product curing in the nozzle?

Leave the nozzle on the cartridge with a little product squeezed out of the tip. It cures into a plug you can pull out next time. If product has cured further down, unscrew the nozzle and dig the hardened sealant out.

The mould keeps coming back — is it the sealant?

Usually not. Recurring mould points to either residue from the previous colony left in the joint, or a bathroom that stays damp. Cut the bead out fully, treat with Rapid Mould Killer, dry thoroughly before resealing, and improve ventilation.