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Primer or Undercoat - What Does Your Timber Fence Need?

Are you planning to paint your timber fences and wondering whether to start with a primer or an undercoat? A few years ago, the decision would’ve been easier with a typical coat of primer, followed by an undercoat and two coats of premium fence paint.

However, with so many options available today, it’s tough to know whether to choose a primer, an undercoat, a self-priming paint or a combination before you apply your topcoat of fence paint. 

A wrong choice could compromise the integrity and appearance of your timber fence, and might end in a waste of your time, efforts and money. 

When it comes to painting your timber fence effectively, making the right choice between primer and undercoat will eventually impact the success of your project.

These two products may seem similar, but understanding their subtle differences is crucial to achieving a long-lasting and beautiful finish for your timber fence.  

At Stock & Noble, we provide the finest fencing solutions to signature property owners in Australia and New Zealand. Our Master Blend collection includes high-quality fence paints. This range, developed by paint specialists after extensive research, also includes special primers designed for both wood and timber. 

With our extensive experience in helping clients achieve stunning and durable painted fences, we’ll provide you with some valuable tips on making the right choice between a paint primer and an undercoat. 

In this comprehensive blog, we will explore the role of primer and undercoat, their key differences, and the best practices for their use. You will have all the information you need to choose the right product before painting your timber fences. 

The Main Difference Between Primer and Undercoat 


What is a Paint Primer? 

Primer serves as a sealant for the timber, creating a protective barrier that effectively prevents water intrusion. This sealing action helps in guarding the timber against rot caused by moisture.  

Beyond this, primer enhances the adhesion of paint to the timber’s surface, making it an ideal choice for new, unpainted timber. However, the timber should be dry and clean before applying the primer. 


What is an Undercoat? 

On the other hand, the undercoat functions as a preparatory coating. It smoothens the surface and provides a consistent base for the final paint. 

Undercoats are often applied on top of the primer or directly on previously painted surfaces, ensuring a smooth canvas for the final paint to adhere to. 


Difference in Colour and Application of Primer Vs. Undercoat 

Paint Primer 

Typically, primer comes in a grey hue, which provides an excellent base for subsequent paint layers, especially lighter colours like white. This neutral colour conceals the timber’s original colour, ensuring that it doesn’t bleed through the new paint. 



Undercoats come in various shades and are particularly useful when transitioning from light to dark colours or vice versa. They help enhance the richness and vibrancy of the final colour, making them invaluable for achieving the desired aesthetic appeal. 


The Difference in Consistency and Cost of Primer Vs. Undercoat 

Paint Primer 

Primer is generally thinner in consistency, allowing it to penetrate the timber’s cells and seal them off effectively. It is available in both oil-based and water-based variants, with costs comparable to undercoats. 



In contrast, the undercoat is thicker and provides a smoother surface for the final topcoat to rest upon. Like primer, it is also available in both oil-based and water-based formulations. 

Both Paint Primer and Undercoat are similarly priced. 


Difference in Quality and Longevity 

Wood Primer 

Primer excels in sealing and moisture-blocking properties, thus extending the lifespan of the paint and, by extension, the timber. Its primary role is to protect the timber, attach the top coat to the timber surface and ensure beauty and long-term durability. 



Undercoats contribute to the longevity of the final paint layers and facilitate a polished, long-lasting finish. They create a cohesive foundation for the topcoat to adhere to, ensuring a professional finish. 

Differences in the working of Primer and Undercoat 


How a Paint Primer Works 

A primer is an essential component in preserving your timber fence and ensuring that paint adheres effectively. Let’s delve into the mechanics of how a primer operates: 

1. Sealing and Adhesion

Primer is designed to create a protective barrier by allowing its solvents to penetrate the timber’s cells. This sealing action effectively closes the cells, preventing external moisture from infiltrating the timber. As a result, it acts as a formidable shield against the detrimental effects of moisture, ultimately enhancing the timber’s longevity. It helps the top coat stick to the timber surface effectively. 


2. Moisture Defense

One of the primary functions of a primer is to serve as a moisture shield. By embedding itself within the timber, the primer guards against moisture penetration, which can lead to timber deterioration and rot. This protective layer serves as a shield against the elements, ensuring that your timber fence remains robust and resilient. 

It’s worth noting that Forever Primer from Stock & Noble is designed to work as both a primer as well as an undercoat. 


How an Undercoat Works 

An undercoat is different in its purpose, primarily used when repainting or transitioning from old to new paint. Let’s unravel the functionality of an undercoat: 


1. Adhesion and Colour Blocking

An undercoat is commonly used when you need to paint over an existing layer of paint. In this scenario, it fulfils a dual role: 


2. Foundation for New Paint

The Undercoat establishes a fresh foundation for the new paint, ensuring a smooth and consistent surface for the final coat to adhere to. It is the adhesive bridge between the old and the new paint. 


3. Colour Blocking

Undercoats are particularly useful in preventing the colour of the old paint from bleeding through the new paint. This feature becomes critical during transitions from light to dark colours and vice versa. 


4. Texture Enhancement

In contrast to primer, an undercoat has a thicker consistency. This thickness allows it to create a smooth and uniform surface for the topcoat to sit on. The result is an improved texture and overall appearance of the final paint job. 


Primer or Undercoat: What Does Your Timber Fence Need? 

The choice between primer and undercoat for your timber fence depends on various factors, including your desired colour transition, the condition of the fence, and the environment in which it is located. Here are some factors to consider: 


1. Are you changing from dark to light colour?

If you’re transitioning your fence from a dark to a light colour, an undercoat can be useful. It acts as a colour blocker, preventing the previous dark colour from bleeding through the new paint. In this situation, you can also consider using Forever primer as an undercoat. 


2. Are you painting a brand-new fence?

For a freshly installed fence, a primer is essential. It seals the timber, providing a protective barrier against moisture. Following the primer, apply two coats of Forever Black or premium fence paint for a durable and attractive finish. 

Please note that as the primer effectively seals the timber, it is crucial to allow your new fence to undergo a natural weathering and drying process, enabling the release of moisture and natural tannins before the primer is applied. Refer to our FAQs for a straightforward test you can do to check whether the fence is prepared for painting or not. 


Pro-Tips for Painting Beautiful Timber Fences  


Choose one range:

Nowadays, some companies offer combined primer and undercoat products, streamlining the application process. 

Opting for a complete range that includes both primer, undercoat and paint within the same brand ensures optimal compatibility and performance between the various coating layers. So, we recommend choosing one brand and sticking to it for all your paint products for the project. 


Consider Primer and 2 top coats:

In the current painting landscape, the standard approach involves using a primer alongside two top coats. This method generally suffices for painting most timber fences, providing beauty and durability. 


Be Cautious of Self-Priming Paint: 

While some manufacturers claim that they have primer infused in the paint and that a single coat of paint is sufficient, it’s always advisable to incorporate a layer of specifically made primer for the most effective and long-lasting results. 


Ready to Choose Between a Paint Primer or an Undercoat? 

In summary, the selection between using primer and undercoat for your timber fence is not a one-size-fits-all decision. Instead, it relies on various critical factors such as the desired colour transition, the current condition of your fence, and the type of wood you are working with. 

Simply put, both primer and undercoat have important jobs in keeping your timber fence safe and looking good. 

Primer works as the first shield, sealing the wood and protecting it from water damage. It also helps the paint stick better, making your paint finish last longer. Using primer, especially on new wood, can extend the life of your fence. 

On the other hand, undercoat really only applies to fences that have already been painted. 

In our experience, 99% of scenarios are best to work with a primer and an undercoat is rarely relevant or required when painting post and rail fences in Australia. 

The next step is to learn how to choose the best paint primer for your fences. It’s good to have a checklist in your back pocket and refer to it as you scan through different primers in the market. 

We understand the choice between various paint products, primers and undercoats could be overwhelming. If you have any questions related to painting your timber fences, speak to a professional. They can help you choose the right products and guide you every step of the way to achieve beautiful fences and create a stunning property. 


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