What is “Bagging” – and Why Would You Need to Do It Before Applying a Long-Life Masonry Coating?

“Bagging” is a term commonly used within the wall-coating and building industry. It describes the application of a relatively strong cement-and-sand slurry to the surface of a wall before the final coating system is applied.

The mix can incorporate SBR (Styrene Butadiene Rubber) along with water to improve adhesion and performance. It is generally applied across the surface using a coarse brush, broom head or, traditionally, a hessian bag – which is where the term bagging comes from.

The exact application method will depend on the condition of the substrate and the finish required.

So, why would you want to do it?

The simple answer is that not every wall provides the ideal surface for a long-life masonry coating.

There can be many reasons why a bagging coat may be appropriate.

Pebbledash and roughcast

Pebbledash and roughcast surfaces can be extremely uneven, with deep depressions between the aggregate.

A bagging coat can help to fill some of these deeper areas and create a more consistent surface. This can reduce the possibility of excessive amounts of coating accumulating in deep recesses during application.

The result can be a more uniform coating thickness and a much more consistent finished appearance.

Brickwork

Some brick surfaces are unsuitable for direct coating, particularly where the bricks are very porous, friable or otherwise provide a poor substrate.

Bagging can help create a more consistent and suitable surface for the coating system.

It can also be used where the intention is to disguise the individual bricks and mortar joints, creating a much flatter, more uniform appearance once the final coating is applied.

Solid walls

On some solid masonry walls, particularly older properties with no cavity, an appropriately specified bagging coat can form part of a system designed to help reduce water penetration through the wall.

The inclusion of SBR can improve the properties of the cementitious mix, helping it adhere to the substrate and resist water penetration.

Importantly, bagging should not be taken below DPC level, as this can interfere with the way moisture is managed within the building.

Previously painted or coated surfaces

Some properties have accumulated several layers of paint and coatings over many years.

Even where the existing coatings are sound, the underlying surface may be uneven. Applying a suitable bagging coat can help flatten and regularise the substrate before the final coating is applied.

This can be particularly important where different textures, repairs or previous coating layers might otherwise remain visible through the new finish.

This effect is sometimes referred to as “mapping” – where variations in the underlying substrate or previous coatings become visible through the finished surface.

Why is bagging sometimes so important?

Bagging isn't required on every property – but where the substrate is unsuitable, it can be an extremely important part of the overall coating system.

A long-life masonry coating can only perform as well as the surface it is applied to.

Preparing the substrate correctly can therefore be just as important as selecting the right coating.

A properly prepared surface can help produce:

• A more consistent coating thickness

• Better adhesion

• A flatter, more uniform appearance

• Reduced visibility of underlying defects

• Less risk of “mapping”

• A more suitable substrate for the final coating system

In some situations, bagging may be unnecessary. In others, it can be an essential part of achieving the best possible result and helping the coating system perform for as long as possible.

Don't overlook the preparation

Bagging is sometimes overlooked because it adds an additional stage – and therefore an additional cost – to a coating project.

However, good preparation is one of the most important parts of any long-life masonry coating system.

If your property has a particularly uneven, porous, damaged or heavily previously coated surface, it is worth discussing whether bagging is appropriate before the final coating is applied.

Your Andura Registered Contractor will be able to assess the condition of the substrate and advise whether bagging should form part of the preparation process.

It may add to the initial cost, but getting the preparation right could make all the difference to the appearance, performance and longevity of the finished coat.