The undercoat plays a fundamental role in a painting project, although it is often underestimated. A crucial step in any painting project, it forms the foundation that ensures not only the longevity but also the aesthetics of the final work. This essential preliminary work, although rarely visible once the project is completed, is the key to the success of a painting project, regardless of the surface (plaster, walls, ceiling, or woodwork).
What is an underlay?
The undercoat is often seen as a simple preparatory step. In reality, it is an essential element for all painting jobs and all types of surfaces before painting the finish. It is designed to prepare surfaces before applying paint, with the simple objective of improving the adhesion of the topcoat and acting as a primer. Using a bonding primer also ensures a uniform and more durable finish.
The composition of the undercoat varies depending on the type of surface to be treated and the desired result.
Indeed, as we presented in a previous guide: there are many undercoats which differ depending on the brand and the type of support on which it must be applied (wall, ceiling, wood, plaster, metal, etc.).
There are even tinted undercoats!
Overall, an undercoat is made up of binders and various additives that optimize its adhesion and resistance.
What is the difference between paint and undercoat?
Unlike topcoat paint, undercoat is not intended to be "visible" on the surface to which it is applied, and serves only as a primer. It has a thicker and less aesthetic consistency than matte, satin, or velvet topcoat paint.
The primer therefore has a real capacity to mask surface defects: such as stains, scratches or color differences.
Since the commercial undercoat is overwhelmingly white, its application is all the more essential on walls that already contain a color - because the undercoat is the product most likely to cover a wall that already contains color.
Its main purpose is to ensure a smooth and even surface for the topcoat, while contributing to the durability of the coating. In addition, some undercoats are specially formulated for specific uses, such as blocking stains, preparing unconventional surfaces or simply to ensure the adhesion of the topcoat in specific conditions (humidity for example) or on specific surfaces (metal or plastic).
The distinction between primer and paint lies in their role: while primer serves as a foundation, preparing and protecting the surface, topcoat paint provides the final color and aesthetics. Now you know, choosing a primer is just as crucial as choosing paint to achieve a professional and lasting result.
The undercoat is applied like a finishing paint: that is, with a brush, roller, or spray gun. Therefore, there is no difference in application method between the undercoat and the paint.
What are the benefits of an underlay?
To fully understand what an undercoat is, it is important to keep in mind the many benefits offered by applying an undercoat in addition to its primary function, which is adhesion.
Using an undercoat before applying paint offers several crucial benefits, both for the durability and aesthetics of the painting project.
First, improving paint adhesion is one of the main benefits of undercoating. By creating a seamless surface, applying a primer coat allows the topcoat to adhere evenly and securely. This enhanced adhesion reduces the risk of paint chipping, blistering, and peeling over time—particularly important in environments subject to temperature or humidity fluctuations.
Next, the primer plays a crucial role in protecting and leveling the surface. It helps hide imperfections such as stains, small cracks, or holes, and creates a neutral background that enhances the depth and purity of the topcoat's color. Additionally, some primers are formulated to resist mildew or block stains, providing additional protection for surfaces exposed to harsh conditions.
In short, applying an undercoat is an essential step to ensure a reliable aesthetic finish for the success of your work. It not only improves the final aesthetic but also protects the painted surface, thus ensuring the longevity of the work accomplished.
To learn more, see our article on what an underlay is for
What are the different undercoats?
Knowing what an underlay is and its role is very important, but did you know that there are several types of underlay?
The various primers available can be selected based on a wide range of criteria: they are distinguished in particular by their use for interior or exterior surfaces. Interior undercoats are formulated to adhere to walls and ceilings, ensuring good adhesion of the finishing paint.
Exterior undercoats are designed to withstand the elements, such as moisture, UV rays, and extreme temperatures. They provide robust weather protection, essential for maintaining the durability of exterior paints.
There are undercoats specifically designed for different materials. For example, undercoats for wood or metal surfaces often require a primer before painting.
To learn more, check out our article on the different types of underlayment
The most common undercoat, however, remains that for materials such as plaster as well as walls and ceilings, although undercoats depending on the type of room also exist.
This is also the underlay that we sell on our website, which is multi-support.
Choosing the right undercoat for the type of surface is therefore crucial to ensuring the success of your painting project. A well-suited undercoat guarantees better adhesion, increased protection, and a superior aesthetic result.
