Silicone, sealant and filling gaps

The tube aisle contains a dozen similar-looking products that behave entirely differently, and using the wrong one guarantees the repair fails.

The distinction that matters

Filler is for gaps and holes that do not move, in surfaces that will be painted.

It sets hard, sands smooth and takes paint.

Flexible sealant is for joints that move — between different materials, around baths, at wall-to-ceiling junctions, around window frames.

It stays flexible and does not crack when the joint moves.

Using filler in a moving joint produces the crack that reappears every year.

Using sealant where filler belongs produces a surface that cannot be sanded or painted properly.

The main types

Silicone is waterproof, highly flexible, durable, and cannot be painted.

Which makes it right for baths, showers, sinks and sanitary applications, and wrong anywhere paint is going.

Silicone contamination also prevents paint adhering nearby, which is why removing old silicone thoroughly matters.

Sanitary grades contain a fungicide, which is worth using in wet areas.

Acrylic sealant, frequently sold as decorator's caulk, is paintable and less flexible and not waterproof.

Which makes it right for skirtings, architraves, coving and internal decorating gaps, and wrong in wet areas.

Polyurethane sealant is very strong, adhesive and durable, used externally and structurally, and harder to work with.

Hybrid polymer sealants are the modern general-purpose option: paintable, waterproof, flexible and adhesive, at higher cost.

Frame sealant for around external windows and doors, formulated for the movement and weather exposure.

Fire-rated sealant, which is required where a fire compartment is penetrated and which is not interchangeable with anything else.

Fillers

Which also vary.

Ready-mixed lightweight filler for small holes, easy to sand, and not suited to deep or structural gaps.

Powder fillers mixed with water, which are stronger and set harder.

Two-part fillers for exterior timber and for anything requiring real strength.

Expanding foam for large voids, which is not a finish and which needs cutting back and covering.

Deep holes need filling in layers rather than all at once, since thick applications shrink, crack and take a long time to dry.

Application

Where most failures actually originate.

Remove all old material completely, which for silicone means a sharp blade and a solvent remover, and clean and dry the surface.

New sealant over old does not adhere and traps water behind it.

Cut the nozzle at an angle to roughly the joint width, starting small.

Mask both sides if a crisp line matters.

Apply steadily and continuously with even pressure rather than in sections.

Tool the bead promptly with a wetted finger or a shaping tool, which forces it into the joint as well as shaping it.

Remove masking tape while still wet.

And leave it to cure fully — which for silicone means keeping the area dry for the stated period, generally a day.

Why sealant fails

The common causes, in order.

Applying over old sealant or a dirty surface.

Applying to a wet surface.

Using the wrong product for the location.

The joint moving before curing, which is why a bath should be filled before sealing.

A joint that is too deep, where the sealant bonds on three sides and tears — deep joints need a backing rod.

And mould, which grows on soap residue on the surface rather than in the sealant, and which is addressed by cleaning and drying rather than replacement in the first instance.

Mould on sealant

The most common complaint.

Where it is on the surface, cleaning and improving ventilation resolves it.

Where it has penetrated, which appears as black staining that does not clean off, replacement is the only fix.

Prevention: ventilate during and after showering, wipe the joint dry, and use a sanitary grade product.

The underlying cause is almost always inadequate ventilation, which is discussed elsewhere on this site.

General information only, not professional advice. Ventilate when using solvent-based products, and use a qualified professional for fire-stopping and structural sealing.