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When To Use a Primer or Undercoat

Using primers and/or undercoats are necessary during the painting process. Priming helps correct the porosity of a surface, ensuring maximum adhesion, so that you get a perfect paint job. In this blog we will guide you through the differences between a primer and an undercoat, and explain why priming is vital for best results. 

Picking the right primer is important, but with a range of primers, undercoats and sealers available it can be confusing. Decorators often use the terms interchangeably but in simple terms, an undercoat is always a primer, but a primer is not always an undercoat. Although similar, both serve quite different functions. Primers act as a foundation for your paint to stick to while undercoats create a flat and level base for topcoats. One easy way to remember is if a surface is painted use an undercoat, if it's new, use a primer.


An undercoat is used to create a neutral base for colour to develop. This is because they work to cover surface imperfections, therefore they are often sanded before recoating. Undercoats (sometimes referred to as mid-coats) exist to be used on coated ie. primed or painted surfaces, particularly wood.

Undercoats are pigmented and best used with enamel topcoats to form an ample look. When applied over a sealer or primer, undercoats work to reinforce and strengthen its function. This foundation creates a tough, resistant barrier to moisture and forms a perfect base for further product.

Best Selling Undercoats


If you are questioning the condition of your wall for painting i.e. if it’s chalky, powdery, stained or highly alkaline, priming is advised. Priming works to hide stains and prevent discolouration. It should always be used on bare wood, this is because the surface is not only porous with grain variations it also absorbs moisture at varying rates, making achieving an even finish difficult.

Primers help in providing a uniform surface for the paint to adhere to. In the case of choosing between an undercoat or primer, primers should always be used when changing from dark to light colours or when transitioning from a glossy base coat. Primers typically bond best to matt or low-gloss surfaces, and each primer is formulated to the specific requirements of that surface.

There are different categories of primers depending on your needs, e.g drywall primer, wood primer, masonry primer and multi-purpose primers. Bonding primers in particular are perfect for more difficult, awkward surfaces while sealers are capable of managing stains in a single coat.

Whatever your painting project the use of primers and undercoats only serve to enhance the completed look. Not only will your paint last longer, but you also won’t have to worry about chipping, peeling or discolouration. Opting for a primer is a fail-safe way to achieve the perfect finish.  

Shop from our extensive range of primers and undercoats from the biggest and best trade paint brands today.

Best Selling Primers

Have a question?

With so many options, it can be difficult to choose which primer and undercoat is right for your project. Our paint experts are here to help, get in touch by calling us on 033 3320 2369 or email [email protected]