9th Dec 2022 -

A layer of undercoat paint is crucial when painting a surface, helping to create a consistent base onto which a beautiful finish can be applied. Within the UK, there is often confusion between undercoats and primers, which are often sold together but are in fact performing different tasks. This quick guide from Building Materials Nationwide aims to clear up any confusion and show the benefits of undercoats, and explore the different types sold within the UK.

What is an Undercoat?

Undercoat paint is intended to create a smooth and adhesive surface, onto which a standard paint can be applied. They bring several crucial benefits to plastered walls, wooden surfaces, and more, allowing easier painting and reducing the possibility of colour bleeding through. If opting to use separate products, an undercoat will be applied after priming and before painting. It can also be applied to an existing layer of paint. Different white undercoats are sold in the UK in order to help paint materials like wood and metal. A solid layer of undercoat paint acts as a flat base for a great-looking finished surface, obscuring and filling imperfections, while also allowing existing paint to be completely covered by a new colour.

What is the Difference Between Undercoat and Primer?

Across the building industry, the terms primer and undercoat are often used interchangeably, and within the UK a lot of undercoats will be primers and undercoats. But in the strictest definition, a primer is an initial coating designed to protect the base material that is being painted, be that brick, wood, or metal. It is also designed to keep moisture away from the base material, avoiding issues like absorption, rust, and rot. For some surface types, both undercoat and primer are vital, such as a wooden panel which requires a primer to avoid water ingress, and an undercoat to allow for an even and attractive finish.

Do I Need an Undercoat on My Painted Walls?

If looking to change colours dramatically, such as from black to white, or bright pink to light blue, colours can bleed through when directly applied on top of one another. Using an undercoat will create a white surface over the existing paint, which can safely be painted over. The same is true of moving between sheens, such as moving from gloss paint to matt paint. If you are looking to paint similar colours on an already painted wall, or to repaint in the same colour, then an undercoat may not be required.

What Types of Undercoat Paints are Available?

There are a variety of undercoat options available in the UK, with water-based and oil-based choices, and others designed around specific surfaces. Almost all undercoats will include primers, meaning that they can fully prepare a surface for painting.

Oil-Based Undercoat

Oil-based undercoat paints are for use with oil-based finishing paints. While these paints were traditionally the choice of professionals, advances in water-based options mean that today many will use both, or have moved to water-based entirely. It’s worth keeping in mind that oil-based undercoat paints will release volatile organic compounds (VOCs) while drying, which can be dangerous. Ensuring adequate ventilation is crucial, with many opting to use water-based paints internally.

Water-Based Undercoat

Intended for use with emulsion paints, water-based undercoats are vital in many cases for a smooth finish. Most of the options within the UK will also function as a primer, effectively offering protection for the underlying material and an adhesive surface for painting in a single coat.

Acrylic Primer Undercoat

This is another type of water-based undercoat that will also include a primer. Depending on the brand it can be suitable for use on wood, masonry, and plaster.

Undercoats for Wood

Undercoats specifically intended for bare wood are made by brands like Dulux, and act as a primer and undercoat for elements like furniture, skirting boards, and stairs. 

Quick Dry Undercoat

For jobs where time is of the essence, quick dry undercoats are available. These will be ready for finishing coats within a couple of hours. Quick dry products are produced by popular manufacturers like Dulux and Crown.

Primer and Undercoat

When shopping in the UK, most undercoat options you find will also work as primers, allowing you to fully prepare a surface with one coat.

Quality Building Supplies Delivered to You

Building Materials Nationwide works with professionals across the UK, saving them time and money by streamlining their supply process. Through our website, we offer a huge range of options for tradespeople and DIYers alike. We also provide trade accounts for customers within the industry, gaining access to a dedicated account manager – a real person you’ll be able to contact via phone or email. Your dedicated account manager will be able to source the supplies you need, from undercoats and primers to specialist equipment, and have it delivered to you at a competitive trade price. Trade account holders can also apply for up to 30 days of interest-free credit.

Sign up for a trade account through our website today, or reach out to us through the form below.

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