If you have ever stood in the sealant aisle of a DIY shop, or watched a tradesperson “run a bead of mastic” around your bath, you have probably wondered whether mastic and silicone are the same thing. They are closely related, they are applied in the same way, and the words are often used interchangeably on site — but they are not identical, and using the wrong one is one of the most common reasons a sealant job fails early. Here is a plain-English guide for UK homeowners.
Mastic and silicone: what the terms actually mean
“Sealant” is the umbrella term for the flexible, gun-applied products used to fill and waterproof joints. “Silicone” is a specific type of sealant made from a silicone polymer. “Mastic” is a broader, more traditional trade word — in UK practice it usually refers to acrylic or hybrid frame sealants, though many trades use “mastic” loosely to describe any cartridge sealant, including silicone. The confusion is understandable, but the two materials behave very differently once cured.
Where mastic (acrylic) sealant works best
Acrylic-based mastic is designed for internal, low-movement joints that stay dry — think gaps around skirting boards, architrave, door frames and interior window trim. Its big advantage is that it can be painted over, so it blends invisibly into decorated walls and woodwork. The trade-off is that it is less flexible and not genuinely waterproof, so it should never be relied on around baths, showers or sinks where it will eventually crack, shrink and let water through.
Where silicone sealant is the better choice
Silicone is the material you want anywhere that gets wet or moves. It stays flexible for years, resists water, and many products contain fungicide to fight the black mould that ruins bathroom joints. That makes it the correct choice for the seal between a bath and tiles, around shower trays, along kitchen worktops and splashbacks, and on external joints exposed to weather. The main limitation is that standard silicone cannot be painted, so colour matching matters — which is exactly the kind of finish detail our bathroom mastic and sealant service is set up to get right.
Key differences at a glance
Durability: silicone typically outlasts acrylic mastic in wet or high-movement areas, often by years. Flexibility: silicone flexes with the natural expansion of baths and worktops, while acrylic is more rigid and prone to splitting. Paintability: acrylic mastic accepts paint, silicone does not. Waterproofing: silicone is genuinely water-resistant, acrylic is only splash-tolerant at best. In short, match the product to the job rather than reaching for whatever is already in the gun.
Common mistakes that make a sealant job fail
The most frequent errors we are called out to fix are silicone applied over old, mouldy sealant that was never removed; acrylic mastic used around a bath where it quickly cracks; and beads applied to damp or dusty surfaces so they never bond. A clean, dry, correctly-prepared joint matters as much as the product itself. If your existing sealant is peeling, discoloured or letting water behind it, it usually needs stripping back and redoing properly rather than a fresh bead run over the top — something we cover as part of our kitchen mastic and sealant work.
Not sure which you need?
If you are staring at a failing joint and cannot tell whether it needs acrylic mastic, silicone, or a full strip-and-reseal, you do not have to guess. Send us a few photos and we will tell you exactly what the joint needs and how we would put it right. Get in touch for a fast, no-obligation quote and we will handle the rest with a clean, long-lasting finish.
