Guide

Silicone vs. mastic: what is the difference?

In everyday UK use, "mastic" and "silicone" are often treated as the same thing. They are not quite, and using the wrong one in the wrong place is a common reason for jobs to fail.

Updated 10 October 2026. General information, not professional advice.

What people mean by "mastic"

Traditionally, mastic was a putty-like, oil-based compound used to seal joints. Today, tradespeople use "mastic" loosely to mean any flexible sealant that fills a joint, and a "mastic man" may apply several different types. Silicone is one kind of sealant. So a mastic man will often use silicone, but not always.

The main types, in plain English

Type	Good for	Watch out for
Sanitary silicone	Baths, showers, basins, kitchen worktops and other wet areas. Stays flexible and water-resistant.	Generally cannot be painted over. Does not bond to old silicone.
Acrylic sealant (often called decorators' caulk)	Gaps in skirting boards, architraves and plaster before painting. Can be painted.	Less flexible and less water-resistant. Not usually the right choice for wet areas.
Hybrid or MS polymer sealant	Many building joints and frames, and some wet areas where a paintable or all-round product is needed.	Check the product label for the uses it is approved for.
Polyurethane sealant	Some external and construction joints.	Usually needs the right primer and conditions. Often a job for a specialist.

Which should I use?

Bath, shower, basin, kitchen sink: sanitary silicone labelled as mould-resistant.
Skirting boards, door frames and small cracks before decorating: acrylic sealant, so you can paint over it.
Windows and external frames: a product made for external use. Ask an applicator, because the right choice depends on the frame and the gap. See our window and door mastic guide.
Anything unusual: read the manufacturer's data sheet, or ask an applicator. Movement joints are covered in expansion and movement joints explained.

Why it matters

An acrylic bead in a shower will usually fail quicker than silicone, and silicone cannot be painted, so a painted-over silicone bead will peel. Matching the sealant to the place saves redoing the job. If your bath or shower sealant is failing, see the signs your bath sealant needs replacing and the reseal guide.

Need a hand?

Tell applicators what you want sealed and where, and they will advise on the right product. Post a job, or read about how to find a mastic man near you and kitchen sealant.

Quick answers

Is a mastic man the same as a silicone man?

Mostly the terms are used interchangeably, but a mastic man may use other sealants as well as silicone.

Can I paint over silicone?

Generally no. Paint does not stick well to silicone. Use a paintable acrylic or hybrid sealant where you intend to decorate.

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