Why should you be preparing walls before painting?

Why should you prepare walls before painting?

Preparation is not the interesting part of painting, but it is what decides whether the finish is smooth and whether the paint lasts. Each preparation step exists because of a specific failure it prevents: cleaning prevents peeling and bubbling, filling prevents cracks showing through, gap filling prevents a ragged line between wall and trim, and sanding prevents flaking.

Put another way, paint does not hide problems — it highlights them. A fresh coat makes an unfilled crack more obvious, not less.

Why do I need to clean walls before painting?

Walls collect dust and dirt unless they are cleaned regularly. Modern paints are made to adhere to nearly any surface, but they stick best to smooth, clean surfaces.

If the surface is dirty, oily or greasy, you risk the paint peeling or bubbling once it dries. If the walls are dusty when you apply the paint, the surface may look mottled or bumpy rather than smooth.

Selleys Original Sugar Soap is a professional-strength formula that powers through tough grease and grime and is safe to use on most washable surfaces without scratching. On walls and ceilings there is no need to rinse, which is why it suits pre-paint cleaning particularly well.

Dilution depends on which product you have. Original Sugar Soap (750 mL, makes up to 30 litres) goes half a cup into half a bucket — 5 litres — of water. Original Sugar Soap Super Concentrate (1 L, makes up to 80 litres) goes half a cup into a full 10 litre bucket. Dilute to suit the task, and use it neat on stubborn or in-ground marks with a non-scratch scouring sponge — a Selleys Wash Up Wiz is ideal for that, and a Selleys Super Cloth for general wiping.

A few cautions: wipe it off aluminium and brass surfaces immediately, rinse it well off glass before it dries, and rinse floors, baths and shower recesses well after use. It has a pH of around 11 and causes serious eye irritation, so rubber gloves are recommended, especially for sensitive skin.

Why do I need to fill cracks in walls before painting?

Painting over the cracks does not work, and the reason is worth stating: a fresh coat of paint accentuates cracks and imperfections, making the wall look messy and unfinished rather than hiding them. New paint reflects light evenly, which is exactly what makes an unfilled defect stand out.

Watch out for cracks, tiny holes from nails and screws, and dents from door handles or furniture. Selleys Spakfilla Rapid handles these — it is ready-mixed, fast-drying, virtually non-shrinking and non-cracking, and needs only one application. It dries in 30 minutes at less than 2 mm and 24 hours at more than 2 mm, and it can be painted with water-based paints once set, or after 24 hours with oil-based paints.

Where the defect is deeper or the surface takes knocks, Spakfilla Heavy Duty is the tougher option, setting in 2–3 hours for 2 mm applications and longer for thicker ones.

Why do I need to fill gaps around skirting boards before painting?

Filling gaps around skirtings, cornices and walls gives you a clean, crisp painted line between the wall and the wooden trim. It also creates a smooth barrier, so the gaps do not collect dust, dirt or insects.

The important word here is flexible. A flexible filler such as Selleys No More Gaps copes with building movement, so it will not crack. A rigid filler in the same joint will — the joint between a timber skirting and a plaster wall moves as timber takes up and gives off moisture, and nothing rigid survives that indefinitely.

No More Gaps Interior Multipurpose is water-based, with a skin time of 20 minutes, a movement capability of ±15% (ISO 9046) and elongation at break of over 300%. Allow around 30 minutes before painting with water-based paint, and 24–48 hours before oil-based paints — longer again for flat and ceiling paints, which contain less binder and flex less. To avoid paint cracking, apply to gaps no larger than 5 mm deep and 10 mm wide at temperatures above 10°C.

It is not suitable for permanent water immersion or constantly damp areas, and not for gaps between tongue-and-groove boards or weatherboards — No More Gaps Exterior is the product for those.

Why do I need to sand a wall before painting it?

Unless the wall is brand new, it probably has bumps, imperfections and old roller lint from previous paint jobs. Sanding gets rid of the rough spots and leaves a smooth surface for the paint.

Sanding also improves adhesion. The sandpaper creates small, rough ridges that give the paint something to grip, which makes it less likely to flake later.

Why do I need to remove flaky paint before painting a wall?

Painting straight over flaky paint gives you an uneven finish that peels or bubbles. Peeling paint can also indicate a lack of adhesion between the wall and the existing paint — painting over it masks the problem rather than fixing it, and the new coat comes away with the old.

Weatherboard paint preparation

Exterior weatherboard needs the same treatment as interior walls: cleaned, sanded, and any gaps filled before repainting. Maintaining weatherboards means removing rough areas and old flaky paint so the new paint has a clean surface to adhere to, which is what makes it last.

Use the exterior products for the filling. No More Gaps Exterior is formulated for gaps around and between weatherboards, eaves, and exterior windows and doors, with UV protection from a titanium compound, ±25% movement capability, 500% elongation at break and an application temperature range of 5°C to 40°C. For maximum durability on bare exterior surfaces, apply a coat of acrylic paint and let it dry before applying the product, and always paint over with a quality exterior paint.

Common questions about preparing walls

Can I skip washing if the walls look clean?

Walls collect dust and dirt whether or not you can see it, and a greasy or dusty surface is what causes peeling, bubbling and a mottled finish. It is the cheapest insurance in the job.

Which sugar soap do I buy?

Either works. Original Sugar Soap (750 mL) makes up to 30 litres and dilutes at half a cup into 5 litres. The Super Concentrate (1 L) makes up to 80 litres and dilutes at half a cup into 10 litres.

Does sugar soap need rinsing off walls?

No — walls and ceilings do not need rinsing. Glass does, before it dries, and aluminium and brass should be wiped immediately.

Will paint hide a hairline crack?

No. Paint accentuates cracks and imperfections rather than hiding them.

What is the difference between a wall filler and a gap filler?

A wall filler such as Spakfilla is rigid and sands flush — right for holes and dents in a single surface. A gap filler such as No More Gaps stays flexible — right for the joint between two different materials that move relative to each other.

Why does my paint crack along the skirting board every time?

Either the joint was filled with something rigid, or the flexible filler was painted before it had dried. Use No More Gaps, keep the gap within 5 mm deep and 10 mm wide, and allow 24–48 hours before oil-based or ceiling paints.

Do I have to sand a wall that is already smooth?

Sanding also keys the surface, which improves adhesion and reduces flaking. Even a smooth wall benefits from a light sand.

What if the old paint is peeling?

Remove it. Peeling indicates poor adhesion underneath, and painting over it simply transfers the problem to your new coat.

Can I use interior gap filler on weatherboards?

No. No More Gaps Interior Multipurpose is not for gaps between tongue-and-groove boards or weatherboards. Use No More Gaps Exterior.

What temperature should I work at?

Above 10°C when filling gaps that will be painted. No More Gaps Exterior has an application range of 5°C to 40°C, and Spakfilla should not be used below 5°C or in wet conditions.

Where do I find the full step-by-step?

See how to prepare your walls for painting for the complete sequence with quantities and drying times.