Undercoat & primer buying guide


Choosing your undercoat and primer

When you’re decorating, think about whether the surface you’re going to paint is new or old. In general, if the surface is new, you should use a primer. If the surface is old, an undercoat works best. Both primers and undercoats help prepare the surface for the topcoat, and while each provides a base to which the topcoat can adhere, they perform slightly different functions. Knowing when and where to use each type of paint will ensure that your surface looks polished, remains sealed and prevents bleeding of colours and corrosion of materials.

What is the difference between a primer and an undercoat?


Primer is a layer of paint that is first applied to seal, bond and provide a strong foundation on to which you apply the topcoat. There are different types of primers for several different surfaces, including drywall, metal and wood. Primer also contains more oil than undercoat and helps seal the surface to prevent subsequent paint from bleeding through and cracking. There are primers that have built-in fungicides to prevent the development of mould, while some can even help prevent nail heads, steel and other metals from corroding.


An undercoat is a type of primer that is applied to fill in depressions and even out surfaces, block underlying dark colours, prevent moisture from seeping into the material and provide a smooth, uniform surface for the topcoat. Since undercoat has more pigmentation, it makes the subsequent paint colour more cohesive and vibrant.

Is a primer or an undercoat better for the surface I am painting?


When painting new surfaces and materials that are susceptible to harsh conditions, a primer should be applied first. A primer works best when it is applied to a flat, low-gloss surface. Primers can also be used when transitioning from oil to latex and from varnish to paint to ensure that the topcoat adheres to the surface. Wood surfaces also require a coat of primer so that the porous material doesn’t absorb the paint, meaning it is sealed and ready for subsequent coats.


An undercoat can be used on surfaces that have already been painted. A good example is when transitioning from dark coloured walls to lighter ones. The extra layer provides a thick film that prevents the previous paint colour from bleeding through. The undercoat works as a filler and adhesion coat between gloss enamels and emulsion topcoats. An undercoat should also be applied on wood, plaster and metal, but only if there is a previous layer of paint or primer. In some instances, some paints combines both the primer and undercoat. Before using a paint that includes 2 in 1, check to make sure it can be applied to your work surface.

How to prepare your doors and windows

  • On your front door, use masking tape to protect areas that you don’t want painted, such as knockers or letterboxes, and unscrew any door numbers.
  • Sand the surface lightly and brush away any dust.
  • Apply an even layer of undercoat onto the door and let it dry.
  • Choose a satin or gloss finish paint in your chosen colour. Paint it on in an even layer, and once dry apply a second coat to finish
  • For windows, brush away debris and wipe down the frame with a sponge.
  • Scrape away any loose, flaking paint and sand to create a smooth surface. Mask off any areas that you don’t want painted.
  • Apply a coat of primer to any bare wood on your window frames.
  • Once the primer is dry, apply a layer of undercoat.
  • Finally, paint on your chosen colour in either a gloss or satin finish.
Top tips
  • Surface preparation is crucial for achieving an overall cleaner and more polished look. When a surface is sanded and cleaned properly, the primer and undercoat perform better, making a better base for the topcoat and giving it an even and vivid finish.
  • It’s also worth thinking about the room temperature and moisture levels before painting. If the temperature is too hot or cold, the paint will not adhere as well. When painting indoors, the ideal temperature of the air, surface and paint is between 15.5 ˚C and 18.3˚C.

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