How to Achieve Smooth Render Finishes with Soft Sand

Achieving a seamless, smooth finish when rendering exterior solid walls can be a challenge. However, with the right materials and techniques, it is possible to create a flawless surface. For this feature, we explore how using soft sand in renders can help create perfectly smooth finishes.

Soft sand has unique properties that make it ideal for blending and finishing render. These qualities allow for troweling and polishing to a uniform texture. We look at the advantages of soft sand over other finishing materials, proper mixing and techniques for great results. 

What is soft sand?

Commonly known as builder’s sand, soft sand has a fine granular texture. Grains range from 0.063mm to 0.2mm in diameter. Unlike sharp sand, its grains have rounded edges that give the sand a smooth feel rather than abrasive. 

Soft sand grains’ composition, shape and size make it ideal for blending and finishing renders, plasters and concrete. When mixed into render, the fine particles of soft sand can fill microscopic gaps. This creates a seamless surface that is easy to trowel to a uniform finish. 

Smooth, rounded grains also help reduce friction and drag while troweling. The specific render mix determines the ratio of soft sand mixes. Below are some general guidelines to consider.

• For cement-based renders, a mix ratio of 1 part Portland cement and 4 parts soft sand is common. You can adjust this within the range of 1:3 to 1:5 cement to sand.
• People often use a 1:3 ratio of lime putty and soft sand with lime mortar render mixes. Ratios between 1:2.5 and 1:4 are common.
• Acrylic-modified renders contain a higher amount of binder. You can mix them with either a 1:3 or 1:2 ratio of acrylic polymer to soft sand.
• Base coats tend to have lower soft sand proportions while finish coats have higher proportions for smoothness.
• Total binder amounts in the mix may need to increase by 10-15% to account for soft sand's lower internal friction.

What are the benefits of soft sand rendering?

When used to finish external renders, soft sand provides several advantages for creating flawless surfaces. Below, explore the key benefits fine soft sand grains offer when blended into render mixes.

1. Easy to use

Render integrating soft sand has a loose, soft, almost silky feel when troweled. This is because of the rounded shape and smooth edges of the grains. It reduces friction within the mix, enabling easy spreading and compression. You need less physical action to trowel the mix to a flawless finished texture.

2. Strong adhesion

The tiny sand grains make the surface contact better by acting like small ball bearings. This allows an incredibly tight mechanical grip and bond to form. Soft sand finishes rigidly adhere to surfaces and resist cracking or detaching even after years of weather and wear.

3. Abrasion resistance

When polished, soft sand particles compact into an extremely dense, tight finish that stands up to surface wear and abrasion. The smooth rounded grains create a surface resistant to marring, scuffing or erosion from wind, rain, impacts and more.

4. Polished appearance

Correctly troweling and compacting soft sand renders create a smooth, glossy surface resembling polished natural stone. This looks clean and contemporary vs traditional rough stucco finishes. Overall, it adds visual value and enhances brick colour.

5. Flexible mixes

Because of its particle size, building sand improves the finish of any render mix. It enhances traditional lime and mortar mix renders, as well as modern acrylic-modified renders.

Is it possible to render a brick wall?

Rendering over brickwork with soft sand finishes creates beautiful surfaces on areas like garden walls and exterior walls. It also offers waterproofing brickwork properties, improves insulation and reduces maintenance in the long run. You need to follow proper rendering techniques. 

Clean thoroughly

Over time, brickwork collects dirt, dust, efflorescence, and other buildups that you must remove before rendering. Wire brushing followed by pressure washing with water is ideal for general cleaning. Acid solutions dissolve residues and require full rinsing after application to effectively remove heavy soiling.

Apply a key coat

A thin base coat of cement-sand render with a coarse 1:4 mix provides the ideal key for subsequent render layers. This key coat forced into the brickwork surface fills voids, anchors to masonry mortar joints and creates a rigid foundation. You can also integrate a brickwork sealer adhesive into the key coat.

Secure render thickness

The first coat of brickwork needs to be thick enough to cover all uneven areas and low spots. A tight 10mm - 20mm thick first coat prevents shadows or lines from showing through finish coats. You should avoid thin applications that are prone to sinking.

Mesh reinforcement

Alkali-resistant fibreglass mesh embedded into the base coat near its centre provides critical tensile strength. This compensates for the differing expansion coefficients of brick and render. The mesh avoids tension crack damage and gives long-term resilience.

Flexible mix

While cement-based renders adhere well initially, they can crack and detach over time as brickwork shifts. For example, a mix of builder’s sand and gravel can reduce crack resistance. The addition of hydrated lime or polymeric binders makes mixes more allowable for brick movement and thermal changes. 

Control joints 

Allowing for movement by installing vertical control joints over existing expansion joints or at edges prevents exterior render cracks. Therefore, careful joint detailing is important. 

How to tackle soft sand render problems

While soft sand delivers exceptional results when used properly, there are some potential downsides to remember. Below, we outline the 4 difficulties that can arise. 

1. Mix design

The lack of internal friction between soft sand's rounded grains mandates increased binder to achieve proper cohesion and strength. Mixes may require the integration of an additional 10-15% cement or lime. This accounts for soft sand's reduced natural adhesion.

Manufacturers will provide specific mix guidelines, but being aware of this consideration can prevent weak or crumbly finishes.

2. Over-polishing

Tempting to burnish and compress soft sand finishes excessively because of their excellent ease of use. However, this can close the surface pore structure completely. It creates a glassy surface effect that has poor vapour resistance. 

This reduces substrate adhesion and encourages later detachment or finish delamination. You should advise moderation when troweling and polishing.

3. Delamination issues

The multiple coat makeup of external render systems needs proper cure time between layers to dry adequately. Premature overcoating or overworking the previous layer's surface can seal in moisture, leading to unsound inter-coat bonding. 

This manifests as cracked, hollow, detached renders months or years later. Allowing each coat to fully cure prevents bond failure.

4. Efflorescence 

Because soft sand finishes are smoothly compressed and non-porous, they can stop the escape of excess moisture from mixes as they cure. This moisture can carry white mineral salts to the surface as it evaporates. It leaves unsightly white deposits on the render, also known as spalling brickwork.

However, this efflorescence usually recede over time as the system fully dries out. Acrylic-modified renders are less prone to this issue.

Can you use other sand types for rendering?

While soft sand is ideal, it is possible to use other sand types for rendering with some key differences in the outcome.