Difference Between Primer, Sealer, And Undercoat


Primer, sealer, and undercoat are three common terms used in painting. Sometimes, It can be pretty confusing when talking about paint jobs. We often use these terms incorrectly without knowing their actual function. Not to worry though, here we’ll discuss the difference between primer, sealer, and undercoat.

What Is Primer?

Primer is the first coat that is applied directly to the surface of the element on which the paint is to be applied. The word “primer” is derived from the Latin word “prim”, which means “first”. A primer coat is applied to any surface to create or to develop adhesion between the surface where the paint is to be applied and the coat of paint.

A primer consists of different binders mixed in a definite proportion. Binders are used to make primer because binders have properties that develop adhesion between the surface and the paint coat so that it easily sticks to the surface.

The binders for the primer should be compatible with the surface on which the primer is to be applied. If the binders are not compatible with the surface, the intended purpose of the primer may not be fulfilled. A primer also helps to prevent the stains from bleeding through topcoats and ruining the finished painting.

Function Of Primer

The functions of primer are as follows:

1. Primer protects the substrate or the layer on which the coats of paint are applied or are to be applied. This is the primary function of a primer coat.

2. The application of primer on the surface develops an adhesion between the surface and the coat of paint applied.

3. It protects and seals the surface, preventing the entry of moisture.

4. It helps to prevent the formation of rust on steel surfaces.

Types Of Primer

There are three main types of primer, depending upon the type of paint. Various types of primer have their compatibility accordingly with different surfaces.

Different types of primers are ads follows:

  1. Oil based primer
  2. Latex based primer, and
  3. Shellac primer

1. Oil Based Primer

Oil based primers are one of the hot favorites and the primary choice of painters. They are versatile in use and they also go well with latex paints. Oil based primers are compatible with almost all kinds of surfaces like wall, wood, metal, etc. So they are used for internal as well as for external surfaces also.

Oil base primers are used extensively and are believed to be ideal for every surface because it covers the stains on the wall, covers the cracks and also prepares the surface for the application of a paint coat respectively.

2. Latex Based Primer

Latex based primers are mostly used for the preparation of dry wall surfaces. Latex based primers are equally effective as oil based primers. It provides uniformity to the surface and also covers the stains on the wall. Latex based primers are cost effective and are easy to use. They cover small pores, cracks, and inequities on the wall and are resistant to effects like yellowness, flaking, and peeling.

3. Shellac Primer

Shellac primer is ideal for application on wooden surfaces. It is the best alternative for wooden surface for sealing the surface and making it a paint ready surface. It covers even the most severe stains on the wooden surfaces and that is why shellac primer is extensively used for it.

What Is Sealer?

A sealer is either a substitute for a primer or used before applying a primer to the base surface. The main purpose of a sealer is to seal off the surface porosity and provide good adhesion between the substrate and the new primer or paint. Some functions of a sealer are similar to primers.

Function Of Sealer

Some of the important functions of sealer are as follows:

1. Provides good adhesion to the substrate.

2. Recondition old or poor substrates that are unfit for painting such as crumbly old concrete tiles.

3. Seal off any surface porosity by creating a water-resistant layer between the substrate and topcoat.

4. Block off stains.

5. Seal waxes in the substrate.

What Is Undercoat?

An undercoat is a coat applied over a sealer or a primer coat. The main function of an undercoat is to provide strength to the existing primer coat and sealer coat. The undercoat is applied to the surface which is previously painted and is to be painted again.

Undercoat prevents moisture penetration into the surface and also helps in improving the bond between the primer or sealer and the topcoat. It provides a base for the topcoat.

An undercoat makes a surface smooth and even colored. It makes the surface ready for the application of the topcoat.

Functions Of Undercoat

1. An undercoat provides a base to the topcoat for its easy and smooth application.

2. It prevents moisture from entering the surface through cracks and holes.

3. It seals the imperfections on the surface properly and makes it even.

4. It provides smoothness to the applied surface.

Difference Between Primer, Sealer And Undercoat

PrimerSealerUndercoat
What It Is?Primer is the first coat that is applied directly to the naked surface.A sealer is either a substitute for a primer or used before applying a primer to the base surface.An undercoat is applied over the primer coat or sealer and underneath the topcoat of the paint.
FunctionProvides proper adhesion and bonding between the surface and the undercoat.Provides good adhesion between the substrate and the new primer or paint.Provides adhesion and bonding between the primer and the topcoat of the paint.
ExampleZinc chromate primer.Polycrylic seal. Rust oleum primer

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