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What is the Difference Between Sealer, Undercoat and Primer?

What is the Difference Between Sealer, Undercoat and Primer?

It can be pretty confusing when these terms start getting thrown around when talking about DIY projects. There actually is a difference between these products, and often then terms are actually used incorrectly. Not to worry though – we’ve broken it down for you!


There’s more to preparation than just prepcoat, with all the cleaning, paint stripping or a vigorous scrub with sandpaper. It’s crucial nevertheless. What is it? A “prepcoat” is actually the term used to describe all “preparation coats”; so primers, sealers and undercoats are all technically prepcoats!

A prepcoat is any coat of paint applied to your substrate to prepare for the application of the topcoat. When painting a new surface, the prepcoat is actually the most important step of a panting project!


A primer is the first coat of paint applied to the substrate. A primer has many purposes, including:

  • Blocking out stains and tannins
  • Stops rust occurring on a metal surface
  • Smooths out imperfections on a rough surface
  • Creates a barrier to stop moisture travelling from the substrate to the topcoat
  • Some primers contain properties that assist to prevent mould and fungus from developing


A sealer is a special type of primer designed to be used with, or in place of, a primer. A sealer prevents the topcoat from soaking into the substrate, and to a degree, can create a water-resistant layer between the substrate and topcoat. There are quite a few different types – like acrylic, polyurethane, or even a specialised concrete sealer for outdoor applications.


An undercoat is used after a primer. It is used to fill any minor imperfections to create a smooth, even-coloured surface ready for the application of the topcoat. An undercoat also assists to lighten a surface when changing from a dark to a pale colour.

Typically, today you can buy a three-in-one prepcoat product that saves time, money and confusion! Berger Premium Triple Prep or Dulux 1 Step™ Acrylic Primer Sealer & Undercoat are great options for your prepcoats.

If there is still a bit of confusion about prepcoats, or you’re unsure what is needed for your next project, head into your local Inspirations Paint store to chat to our friendly and knowledgeable staff!