Primers, Sealers and Undercoats

So you have decided to give your home a new coat of paint and while looking for paints, you come across this terms and wonder what they do. Firstly, I would like the congratulate you on your brave decision to re-paint your house. In order to ensure your newly painted house will look as good as it can be, it is important to have an understanding of these terms. Not to worry as even professional painters are confused by these words “primers”, “sealers” and “undercoats” are used interchangeably. In this article, we will help you clear some of the doubts you may have.

What is a Primer?

We will first have to take a look at what a primer does.

A primer is a coat of paint that is applied directly to the bare substrate. Its main functions are to:

1. Provide excellent adhesion to the substrate (wall) for the new paint.
2. Provide protection to the substrate (wall) until it can be top-coated.
3. Prevent moisture reaching the substrate (wall)
4. Protect the substrate. Some wood primers have fungicide to prevent mould-growth and anti- rust properties in metal primers.

Primers are usually pigmented to allow painters to know that a wall has been primed. Also they contain a higher amount of pigment volume concentration (PVC) which results in a slightly “rough” finish after drying. This “roughness” provides for better adhesion for the subsequent coats of paint.

What is an Undercoat?

An undercoat is designed to fulfil the following roles:

1. Provide a base for the topcoat to adhere to.
2. Provide a film to prevent moisture from getting to the substrate (wall).
3. Even out small imperfection on rough surfaces.

Undercoats generally have the role of filling surface imperfections and preventing moisture. They have high pigment volume concentration (PVC) to fulfil this role. Steel primers often contain pigments such as micas and talc powder which work well to protect against moisture.

What is a Sealer?

A sealer can be seen as a specialised primer. A sealer can be used prior to, or in place of a primer. Sealers are designed to:

1. Provide good adhesion for the subsequent coats.
2. Recondition poor substrates such as old, cracked walls.
3. Seal off surface porosity to prevent subsequent coats from sinking in which could lead to a “non-uniformed” finish.
4. Prevent stains.

The composition of sealers are quite varied as sealers carry out many different roles. Unfortunately, the naming of sealers and primers can be rather loose among products. In this case, it is important to know what you want to do with the sealer/primer and check against its uses on the product label.

General Rules

1. An undercoat is a type of primer, but a primer is not always an undercoat.
2. If it is a new surface, use a primer. For surfaces that have been painted, use an undercoat.
3. A sealer can be seen as a specialized primer.
4. Always use primers, sealers and undercoats from the same brand as your top-coat to ensure maximum adhesion. While most brands claim their primers and undercoats can work with any paint, they general do testing only within their product lines.

Through this article, we hope we have given you a better understanding of 3 different products. With this new knowledge, you are better prepared to undertake that painting project and utilize these products more effectively.

Wayne represents Turner’s Painting, a five star rated Long Island painting company specializing in residential painting services, by exterior and interior house painters. If you need professional painters for work at your home, contact Turner’s Painting today.



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