Silicone Between Glass Splashback and Caesarstone Benchtop
What can I use to replace mouldy silicone between a glass splashback and a Caesarstone benchtop?
Use a neutral cure silicone, and do not lay it until the old bead is completely gone and the joint has been treated for mould and dried. Selleys names Wet Area Speed Seal as ideal for this particular pairing, because a neutral curing product suits both the glass splashback and the stone benchtop. If Speed Seal is not on the shelf, the other neutral cure option in the wet area range is Selleys Wet Area Silicone, which is also a neutral cure product and is listed for use on glass.
The product choice is the easy half. The reason your two-year-old bead went black behind the sink is that the joint sits in the wettest, most food-soiled spot in the kitchen, and once a mould colony is established in silicone it has to be cut out — not cleaned, not sealed over. Selleys is explicit about this: a well established mould colony can penetrate a new seal, even if the new product has mould inhibitors.
Why neutral cure, and not any silicone off the shelf
Silicone sealants cure by one of two chemistries. Acetic cure silicones release acetic acid as they set — that is the vinegar smell — and they are the default for plain glazed tile. Neutral cure silicones cure by reacting with atmospheric moisture without releasing that acid.
Engineered stone benchtops are the classic case for reaching past the acetic product. Selleys' recommendation for a glass-to-stone joint is a neutral cure product, and that is the single most important thing to get right on this job. Glass itself will take either chemistry; the stone is the reason for the rule.
Because Caesarstone and other engineered stone brands are not listed by name on the Selleys technical data sheets, it is worth checking your benchtop manufacturer's own care guidance as well, and applying a small test bead in a spot you cannot see before you run the full length.
Step by step
- Cut the old bead out completely. Use a sharp blade or a silicone removing scraper and run it along both edges of the bead so it lifts away in long pieces. Long strips come away far more cleanly than short stabs.
- Soften and remove the residue. A thin film always stays behind, keyed into the surface, and fresh silicone will bond to that film rather than to the glass or stone. Selleys Sealant Remover handles this: scrape or cut the old sealant back to a layer less than 5 mm deep, spread at least a 5 mm thick layer of remover evenly over it, leave it for at least 10 minutes, then scrape the softened material away. A second application is sometimes needed. It is suitable for use on glass, ceramic tiles, porcelain, grout, most metals, laminate, masonry and most painted surfaces.
- Treat the area for mould. Selleys nominates Selleys Rapid Mould Killer for this step. Patch test first — staining can occur on some matt finish tiles or in hard water areas, and it is not suitable for fabric, mirror tiles, metallic surfaces, or older baths and basins.
- Dry the joint completely. Ensure the surfaces are fully dried after treating with Rapid Mould Killer. This is the step people rush and it is the one that most often costs them the job.
- Wipe with mineral turps. This clears residual cleaner, greases and grime, and it is also the surface preparation the silicone's own instructions call for. Let it dry completely.
- Apply the new bead. Cut the nozzle at 45 degrees, fit it to the cartridge, load a caulking gun and apply with steady pressure, making sure the sealant is pushed into the joint rather than laid on top of it.
- Tool it within 5 to 10 minutes using a spatula dipped in detergent water. Clean up with turps before the product skins.
How long before you can use the sink again
This depends on which product you end up with, and the difference is substantial.
Selleys Wet Area Silicone cures in 72 hours. That is the figure on its technical data sheet, and it is the point at which the seal has developed its full properties. The bead should not be disturbed in the meantime.
Wet Area Speed Seal is the fast option Selleys names for this pairing — it is the reason the product exists. Follow the cure and re-use times printed on that pack, since they are what apply to the product actually in your hand.
In practice, the sink behind a splashback joint is hard to keep dry for three days. If that is a problem, the fast-skinning product is the sensible buy; if you can wash up elsewhere over a weekend, either will do.
Wet Area Silicone at a glance
| Property | Typical result |
|---|---|
| Technology | Neutral cure silicone, 100% silicone sealant |
| Colour | Matt white |
| Packaging | 420 g cartridge |
| Shore A hardness | 30 |
| Elongation at break | 450% (ASTM D412) |
| Service temperature | −40 °C to +150 °C |
| Cure | 72 hours |
| Tooling window | 5–10 minutes from application |
| VOC content | <50 g/L |
Indicative data only, not intended for the preparation of specifications.
Limitations worth knowing before you buy
- Not paintable. Complete any painting 7 days before applying the sealant.
- Not suitable for use on PP, PE or PTFE.
- Not suitable for aquariums, or for food or drinking water contact.
- Not suitable for permanent water immersion. A splashback joint gets splashed, which is fine; a joint that sits under standing water is not what this product is for.
- Storage. Store in a dry place below 30 °C and use within 6 months of opening.
- Sealant Remover may affect some surfaces — test compatibility before use, and do not leave it on porous materials or acrylic-painted surfaces for extended periods. It will not remove staining caused by old penetrated silicone oils.
Keeping the new bead clean
The joint behind a kitchen sink fails first because it collects food residue, detergent film and standing water. Selleys' own guidance for its wet area silicones is blunt about it: the sealant must be cleaned regularly to avoid build-up of soap scum. Wiping the bead dry after washing up does more for its life than any product upgrade.
Common questions
Can I just seal over the mouldy silicone?
No. An established mould colony can penetrate a new seal even where the new product contains mould inhibitors, so the black spotting comes back and you end up removing twice as much material.
Why does the bead behind the sink go before the rest of it?
That is where the water, food residue and detergent collect. Mould needs a food source, and soap scum and grime supply it.
Which is better for a stone benchtop, acetic or neutral cure?
Neutral cure. That is what Selleys recommends for a joint between a glass splashback and a stone benchtop.
Is Caesarstone specifically approved?
No Selleys technical data sheet lists engineered stone brands by name. The published substrate list for Wet Area Silicone covers ceramic, glazed and porcelain fixtures, metal, copper and brass fittings, powder coated aluminium, aluminium, plastic laminates, rubber, glass, stainless steel and concrete. Check your benchtop supplier's guidance and test a small area first.
How long do I have to smooth the bead?
Five to ten minutes from application, using a spatula dipped in detergent water. After that the surface starts to skin and tooling tears it.
What removes the last film of old silicone?
Selleys Sealant Remover, spread at least 5 mm thick over sealant that has already been cut back to less than 5 mm, left at least 10 minutes, then scraped off. Clean residue and tools with mineral turpentine.
Do I have to use a mould killer if the joint only looks a bit grey?
Yes, treat it. Selleys' instruction for this job is to treat the area with a mould killing product after the old sealant is completely removed, because what you can see is the top of a colony that has rooted into the joint.
Can I speed the cure up?
Not with heat or dry air. These are moisture curing silicones, so they need atmospheric moisture; normal room ventilation is what helps.
Can I paint the bead to match the benchtop?
No. Wet Area Silicone is not paintable. If painting is involved anywhere nearby, do it 7 days before you seal.
What if the joint is between glass and a stone cooktop surround rather than a sink?
Same chemistry decision, but heat becomes a factor. Wet Area Silicone's service temperature range is −40 °C to +150 °C; check that against the surface temperatures your appliance actually reaches.
Where can I read the full removal and resealing method?
See how to remove old sealant and how to replace old silicone.
What is the original question addressed on this Selleys page?
The page addresses the question: 'What can I use to replace silicone that has gone mouldy between a glass splashback and a caesarstone benchtop?' The person notes that the silicone sealant between their glass splashback and benchtop has gone mouldy in the area directly behind their kitchen sink, and it had been installed for around two years.
What sealant product does Selleys recommend for sealing between a glass splashback and Caesarstone benchtop?
Selleys recommends PRO-TRADE Glass Sealant, a 100% neutral cure silicone with excellent weather and UV resistance and a matt finish. It is specifically suitable for glass applications, has strong adhesion to glass substrates, and is available in clear and black colors in 300g and 400g cartridge sizes.
Is there an alternative sealant option for this glass splashback and benchtop application, especially where moisture resistance is a priority?
Yes, Selleys Wet Area Silicone is another suitable option. It is a 100% silicone sealant that delivers superior waterproof and mould-resistant sealing, guaranteed for 25 years, and works effectively on glass materials, making it ideal for kitchen applications exposed to splashes and moisture.
How should the glass and benchtop surfaces be prepared before applying new silicone sealant?
Surfaces must be clean, dry, and free of dirt, grease, oil, or water. The glass and benchtop surfaces should be cleaned with mineral turpentine and allowed to completely dry before applying the sealant.
What is the correct application technique for the sealant between the glass splashback and benchtop?
Cut the nozzle at a 45-degree angle and use a caulking gun to apply the sealant with steady pressure, ensuring the product is pushed into the joint. The sealant should then be tooled with a spatula dipped in detergent water within 5-10 minutes of application.
How long does the sealant take to cure, and what should be avoided during this time?
Both PRO-TRADE Glass Sealant and Selleys Wet Area Silicone cure in approximately 72 hours. During this time, the seal should not be subjected to water or movement until it is fully cured.
Can the silicone sealant be painted over after application?
No, sealant cannot be successfully painted over in most cases. If painting is planned, it should be completed 7 days prior to applying the sealant.