How to pick the right sealant for the job

Walk into any builder's merchant and you'll find a wall of sealant tubes that look, at a glance, almost identical. Same size, same general shape, different coloured labels. It's easy to assume they're broadly interchangeable, and that one silicone is much like another. But the chemistry and intended environment behind them are very different, and that difference is what determines whether a bead lasts for years or starts to fail within months.

Understanding what separates one product from another doesn't require a chemistry degree. It just requires knowing what the job is actually asking of the sealant, and matching the product to those demands.

Most silicones fall into one of two camps: acetoxy or neutral cure. The difference is in how they cure, and it has real practical consequences.

Acetoxy silicones release acetic acid during curing. That faint vinegar smell when you open a fresh tube is the giveaway. They bond extremely well to smooth, non-porous surfaces like glass and ceramic tiles, which is why they've long been the go-to chemistry for sanitary applications. On the right surfaces, in the right environment, they perform extremely well.

The limitation is that acetic acid can react with certain materials. On metals like copper, lead or galvanised steel, it can cause corrosion or interfere with adhesion. On some natural stones, it can cause staining. If your job involves any of those materials, acetoxy isn't the right choice regardless of what else the product offers.
Neutral-cure silicones avoid the acidity entirely. They cure through a different chemical reaction and are compatible with a much wider range of substrates, including metals, masonry, natural stone, and coated surfaces. They're often the standard choice for exterior work, construction joints and any application where acetoxy chemistry would cause problems. The trade-off, historically, has been slightly slower curing, but for most jobs that's a minor consideration against getting the substrate compatibility right.

The distinction between interior and exterior silicones goes further than simply whether a product can handle rain.

Interior silicones are formulated for stable conditions, including consistent temperatures, more limited movement, and protected environments. A bathroom or kitchen silicone is designed to perform in those conditions, and it does so very well. It will typically carry fungicide protection to resist mould, and it's optimised for the surfaces and cleaning products it'll encounter indoors.

Put that same product outside, and the demands change entirely. UV exposure degrades some silicone formulations over time. Seasonal temperature swings create far more expansion and contraction than an interior joint ever sees. An interior-grade product simply isn't built to cope with that level of movement and weathering, and performance will suffer accordingly, often within the first year or two.

Exterior-grade silicones are built for those conditions. They're formulated to stay flexible through wide temperature ranges, resist UV degradation and cope with the kind of joint movement that seasonal changes produce. They're typically neutral-cure, which also makes them compatible with the metals and masonry that exterior applications commonly involve.

On the other hand, using an exterior silicone indoors will generally work from a performance standpoint. But you may lose the specific fungicide benefits of a true sanitary-grade product, which matters in bathrooms and kitchens where mould resistance is part of the job.
 

Sanitary silicones are often thought of simply as "bathroom sealant," but the formulation does a specific job. The fungicide content is there to resist mould growth in conditions that actively encourage it — warm, damp, poorly ventilated spaces where a standard silicone would quickly discolour.

That fungicide protection only works if the product is used as intended. Sanitary silicones used outside lose much of their purpose. The weather resistance isn't there, and the fungicide, while still present, isn't what the product was optimised around. Equally, a standard exterior silicone used around a bath or shower surround will lack the mould resistance the environment demands.

Roof and gutter products are purpose-built for a specific set of conditions, including lap joints, metalwork, emergency weather repairs, and they do those jobs well. But they're not interchangeable with building or frame silicones. They're not designed for structural movement, long-term immersion or use as a perimeter sealant. Using them outside their intended application is one of the more common causes of premature failure.  

Before reaching for a tube, it helps to run through a few quick checks:

• Is the substrate non-porous and smooth (glass, tile, glazed ceramic)? Acetoxy is likely fine
• Does the job involve metal, masonry or natural stone? Go neutral cure  
• Is it going outside, or somewhere exposed to significant movement or weather? You need an exterior-grade product
• Is it a wet room or bathroom? A sanitary-grade silicone with fungicide is the right call
• Is it a roof or gutter repair? Use a product designed specifically for that — don't substitute a frame or building silicone

No sealant can perform properly in conditions it was never designed for. Taking two minutes to confirm the right product before the job starts is a habit that pays for itself every time.