Primer vs. Undercoat

Primer vs. Undercoat – When to Use Which Type of Prep Coat

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An essential part of the painting process is the use of primers and undercoats; however, more often than not, some novice painters will have questions such as, “what does primer do?”, or “what is undercoat?”. Fear not! We have provided all of the information you would need regarding the long-standing debate of primer vs. undercoat. Below, we have provided a complete guide that explains the similarities and differences between these popular products, so let’s get started!



What Is Primer?

Before we can dive into the primer vs. undercoat debate, it is essential to understand what primer is, how it is used, when it should be used, and the benefits of using it. Primer is an incredibly important product that is used in the painting process, but we recommend that you keep reading to find out more.


Definition of Primer

Primers and undercoats should not be used interchangeably, but a good rule of thumb is that all undercoats are primers, but not all primers are undercoats. This is a paint compound that improves the adhesion of finishing paint when applied alone. It is intended to adhere to surfaces and generate a binding layer that prepares the outer surface for painting.

A primer, unlike paint, is not meant to be used as a permanent finish and will instead have enhanced filling and binding capabilities.

Primers are used for a variety of reasons. Some primers are developed specifically for porous materials like concrete and wood. On the other end of the scale are primers developed to adhere to smooth and glossy surfaces like metal and tile. Primers are essential for exterior surfaces that will be exposed to extreme weather, or any material that will face excessive moisture or heat. Primers can also be used on damaged surfaces to provide an even base, or before painting a lighter color over a dark finish.

Porous Surface Primer


Types of Primer Available

As is the case with most painting products, there are a few varieties of primers on the market. These fall under three categories, latex-based, oil-based, and shellac primer, with each type possessing particular qualities that will make it easier for you when working on certain surfaces.

Below, we have provided more information on the different types of primer products and this will allow you to make an educated decision instead of wondering, “can I use undercoat as a primer”.


Latex-Based Primer

Water-based latex primers are suitable for preparing raw drywall for painting. They tend to be less brittle and more flexible as opposed to oil-based primers, therefore being less vulnerable to flaking and splitting. They are also useful for priming softwood, galvanized metals, concrete, and brick surfaces. Latex primers are useful for drywall since they smooth out the outer layer of the wallboard as well as any joint compound that has been put to it, as well as any patched or repaired sections. They can conceal and seal in prior small stains from wax, lipstick, smoke, and so on.

Primer vs Undercoat on Drywall


Oil-Based Primer

For many years, oil-based paints and primers have been the industry standard. These primers are compatible with both oil as well as latex paints, which makes them extremely versatile and adaptable to a broad range of surfaces. Oil-based primers work well on wood and steel, along with other metals, and surfaces that have been painted, such as exterior and interior walls.

Some oil-based primers do a great job of eliminating stains and preventing them from penetrating your new layers of paint. They are effective in removing ink, water, and nicotine stains from your walls.


Shellac Primer

Shellac has been commonly used to seal wood and various other materials for generations. Shellac-based primers are excellent stain-blocking compounds for interior painting tasks and work exceptionally well on smoke and water damage on different surfaces. They are also great at keeping regular water, smoke, rust, stains, and even wood tannins, from leaking through fresh paint. They are fast-drying and very sticky products that work well on plaster, metal, wood, and even plastic. They can also be used with oil-based as well as latex paints.

Primer vs. Undercoat for Smoke Stains


Benefits of Using Primer

Since we are comparing the benefits of primer vs. undercoat products, it is important to have a clear understanding of the benefits. Sure, certain scenarios will allow you to completely skip the priming phase; however, in most cases, you will need it. Below are some of the benefits that come with using primer.

  • Perfectly optimizes the surface for paint, requiring fewer coats of paint
  • Covers imperfections in the surface from repairs
  • Seals pores on unpainted wood and drywall surfaces
  • Allows paint to adhere to the surface
  • Blocks odors and stains
  • Allows you to paint on a neutral surface

By delivering these benefits, Primer saves money and effort. It is less expensive than paint and aids in covering the surface and addressing a variety of difficulties. After priming, the job requires fewer applications of top coat paint as well as less time to complete. So, if you are wondering, “is undercoat the same as primer?”, there are a series of benefits that say otherwise. 


When to Use Primer

After you have learned about the advantages of both exterior and interior wall primers, you will need to understand when to apply them. So, do I need primer and undercoat on bare wood? Take a look at the examples below to find out more.

Primer for Smooth Surfaces


Repaired Drywall

To repair drywall, spackle or joint compound is generally used instead of replacing the damaged section. Patches are then formed along the wall. Since these patches are colored differently than the existing paint, they will stand out if you decide to paint over them without priming.

Since priming converts all surfaces to a neutral white, these defects become unnoticeable. Choosing the appropriate paint color is critical to getting precisely what you want. Additionally, selecting the best grade of wall primer is critical.


Repainting Oil-Based and Glossy Surfaces

The new paint needs a surface to adhere to, and it will not function if you paint over oil-based as well as glossy paint. A bonding primer would prove to be beneficial since it will cling to the surface, providing an outer layer. This layer may be adhered to by the top coat of paint.

Use Primer on Oil-Based Paints


Drastic Color Changes

In the event of a drastic color change, painting over the current color will not result in the proper shade of the new topcoat. The new and old colors will be combined, and the final color will be a blend of both. However, by applying the wall primer, you will acquire the proper hue since it will return the outer layer to white and let you start afresh.

Applying a primer is especially crucial when transitioning from a dark to a light shade.


Unpainted Wood and Fresh Drywall

You could be asking yourself, “do I need primer and undercoat on bare wood?”. The reality is that these surfaces are extremely absorbent and a coat of paint fades fast as it gets absorbed into the porous material. As a result, multiple coats are required to adequately cover the region. It makes no sense to waste expensive paint on material absorption. Primer is inexpensive because it plugs pores and prepares the surface for painting.

Primer for Raw Wood



What Is an Undercoat?

To accurately determine the victor of the primer vs. undercoat debate, it is essential to have a clear understanding of what both of these products do. So, is undercoat the same as primer? Below, we will find out everything that you would need to know when it comes to using this product and more.


Definition of Undercoat

An undercoat serves to produce a neutral basis for the development of color. Since they attempt to conceal surface flaws, they are typically sanded before recoating. Undercoats are available for use on primed as well as painted surfaces, notably wood.

Undercoats have color and function best when combined with topcoats to create a full effect.

Undercoats help to enhance and reinforce the purpose of a primer or sealer when put over it. This foundation offers a strong, moisture-resistant barrier and serves as an excellent basis for subsequent products.


Types of Undercoats Available

There are a range of undercoat alternatives available, including water-based and oil-based solutions. This also includes those that are tailored to certain surfaces. Nearly all undercoats contain primers, which means they may completely prepare an area for painting.

Range of Undercoat Paints


Oil-Based Undercoat

Oil-based undercoats are intended to be used in conjunction with oil-based finishing paints. Although these paints were usually the preference of professionals, developments in water-based alternatives mean that many now use both or have switched entirely to water-based products.


Water-Based and Acrylic Primer Undercoats

Water-based undercoats, designed for use alongside emulsion paints, are critical in a variety of situations for an even finish. Most alternatives available will also serve as a primer. This is an additional water-based undercoat that will incorporate a primer. It may be used on wood, stone, and plaster based on the brand.

Undercoat Paint for Wood


Benefits of Using Undercoat

To properly understand what is undercoat used for, it is important to understand the benefits that it provides. The most effective painting finishes are obtained when the surface to be painted is properly prepared, which is part of the procedure. Undercoat paint provides a series of benefits, such as the following.

  • Provides an even coverage
  • Hides all small imperfections
  • Protects the surfaces from moisture and other external elements


When to Use an Undercoat

Colors can bleed through when immediately spread on top of one another while changing colors abruptly. Using an undercoat creates a white surface over the current paint that may be painted over securely. The same is true when switching between sheens, such as from satin to gloss paint.

Undercoat for Changing Paint Sheen

If you want to paint similar colors on an existing wall or repaint an identical color, an undercoat would not be necessary. So, do I need undercoat on painted walls? No, but you would need it on top of a primer.



Differences Between Primer and Undercoat

Primer, as well as undercoat products, are used to prepare walls for painting, although they have distinct functions. They are, in essence, two parts in a larger operation. A decent rule of thumb is to apply primer when painting a bare surface. Use undercoat if the foundation has already been primed or painted.


Key Differences in Purpose, Composition, and Application

The phrases primer and undercoat are frequently used interchangeably in the construction business, and many undercoats are both primers and undercoats. A primer, on the other hand, is an initial layer meant to preserve the foundation material that is being painted. It is also intended to keep moisture from entering the foundation material, eliminating problems like absorption, corrosion, and decay.

Function of Primer vs. Undercoat

Some surface types require both an undercoat and a primer, such as a hardwood panel, which needs a primer to prevent water penetration, and an undercoat to achieve a uniform and beautiful finish. The application of both of these products is the same; however, undercoats need to be applied on top of the primer, unless it is a two-in-one product. A common question that generally pertains to this is, “do I need undercoat on painted walls?”. Yes, you do.



Choosing the Right Product

For the paint to do its job effectively, the necessary primers and undercoats must be used. The undercoat is frequently used to seal untreated surfaces or prepare them for painting.

The undercoat will help to erase the color beneath if you are moving from a dark to a light color. This will enable your selected topcoat to reach unrivaled color depth.


Factors to Consider When Selecting Primer or Undercoat

Below we have provided a list of factors to consider when choosing primers and undercoats. Each product will allow you to get the best results and it will save you money as you will not need to redo the surface any time soon.

  • Metal: Metalwork that has been previously primed or painted and is in good condition can be painted without extra priming. However, it is a good idea to first sand the surface using a medium grit sandpaper. New radiators are often delivered primed. Nevertheless, we recommend using one coat of water-based primer before applying two coats of your preferred topcoat.
  • Wood: Thoroughly seal new or resinous wood with a coat of wood primer. This will keep any resin from penetrating the topcoat of the selected finish.
  • Walls: Use a wall primer for patchy or porous walls and fresh plaster, particularly skimmed ceilings and walls. This will fill up the surface’s porosity and produce a homogeneous basis for painting.


Tips for Making the Best Choice for Your Project

It is essential to evaluate the surface that will be painted when selecting a primer or undercoat. Various products are developed to suit different surfaces, including wood, concrete, or metal. To achieve optimal adhesion and longevity, use a product that is suitable for the surface that needs to be painted.

Choosing Best Primer and Undercoat

Aside from the type of surface, the type of paint utilized must also be considered. If you are using an oil-based paint, for instance, you will require a primer that is suitable for oil-based paints. Water-based paints are the same.



How to Apply Primer and Undercoat

Applying a primer or undercoat is not a complicated process. Since this is part of the preparation phase of your project, the application does not need to be perfect, but you will need to ensure that you apply the product as evenly as possible.


Step-by-Step Instructions for the Application of Each Product

Before we get started, keep in mind that each surface will require preparation of its own. This includes sanding, repairing holes and cracks, as well as general cleaning. Once you have completed the necessary steps, you can start applying the product.

  1. Begin by stirring the tin.
  2. With your paintbrush, apply the primer. If you are working with wood, it is important to apply the product in the direction of the grain.
  3. Once the product has been applied, wait two hours and then it will be ready to paint.


By understanding that the primer vs. undercoat debate is futile and that both products have their respective uses, you should be able to get the best out of both of them. Primer and undercoats are an important part of the painting process and they will allow you to get the best results. Good luck with your next painting project, and do not forget to apply what you have learned today!




Frequently Asked Questions


What Is the Difference Between Undercoat and Primer?

It is common to confuse primers and undercoats with one another since both of these products can be used as a base layer for the paint. Undercoats would be used on previously painted surfaces, whereas primers create a consistent and smooth foundation for the next layer of paint to adhere to. While this is the difference between undercoat and primer products, there are many more differences that we have mentioned above.


What Does Primer Do?

Primer products serve a variety of purposes such as preventing mold and different funguses from growing. Furthermore, they create a barrier to prevent moisture from reaching the topcoat, smoothing out imperfections that are commonly found on rough surfaces, as well as blocking out tannins and stains.


Can I Use an Undercoat as a Primer?

No, you cannot. Undercoats are always meant to be used after the primer or if you are planning to paint a surface that already has a coat of paint on it. However, we do not suggest using undercoat products as a primer, as they will not provide you with the same benefits that are needed to maintain proper protection.

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