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Primer Vs Undercoat

For a lot of people these two terms have become interchangeable, but is that correct? What actually is the difference between primer and undercoat? Well, let’s take a look and find out once and for all.

The easiest way to remember this is that an undercoat is always a primer, but a primer is not always an undercoat.

What I mean by this is that undercoats are a subset of primers. So while an undercoat will always be a primer there are also lots of primers that are not undercoats.

What is a Undercoat?

An undercoat is a specialist paint used before your main paint is applied. Its main job is to provide a smooth, even surface for the topcoat to go onto.

They normally have a bit of “build up” themselves which helps to give the finished paint a thicker, fuller look.

That same “build up” also helps to hide small surface imperfections. Usually, undercoats are associated mainly with wood.

What is a Primer?

A primer is a paint that is applied to your surface to prepare it for being painted. There are lots of different primers with different uses.

Cover Primers

These primers are intended to cover dark colours or stains. They block the colour from showing through in your final finish.

Often they also stop the colour from “leaking” through to the topcoat. This can be a problem with things like graffiti where over time the paint from the graffiti leeches through to the topcoat.

Etching Primers

These primers are sued mainly for adhesion. They tend to be used on glossy shiny surfaces where normal paint would have a tough time sticking.

These primers etch themselves into the material, giving really good adhesion. They can then be painted over with any regular top coat.

Think of these primers as a way of painting surfaces that would otherwise be out of bounds for regular paint.

Sealing Primers

This is a type of primer that is commonly used on porous surfaces. They work to seal the surface so that the topcoat is not absorbed into the material.

Specialist Primers

As well as all of the other primers I have listed above there are countless specialist primers.

There are primers that stop salt from coming through on masonry, primers that are purpose made for use with certain metals and even stabilising primers that can be used on flakey surfaces.

The list is almost endless, if there is a specialist paint job out there then there is usually a primer fit for purpose.


An undercoat is usually used to build up the surface to allow the best looking finish possible. They are commonly used on surfaces that have already been painted before rather than on brand new surfaces.

The main goal of an undercoat is to get the best top coat finish possible.

Primers commonly have specialist purposes. They are often used to help increase the adhesion of the finish coat to the painted material. For this reason, there are tons of varied primers for certain uses.