interior wood primer and undercoat

How to Choose Primers and Undercoats

Do you want to renew your paints but the surface is damaged, porous or damp? To ensure a nice finish there is only one solution: primer paint. First coat for plasterboard or special for damp walls. We tell you all about primer products.

Why use a primer or base coat

The primer (undercoat) is the first coat to be applied on a good, dry and clean substrate or surface before applying the finished paint. Although we are often tempted to skip this phase (what laziness!), its application is nevertheless essential. 

Indeed, the primer, sealer or sealer, offers many advantages such as better adhesion, easier application, longer duration of the final result; and it also serves to fix (it stabilizes a porous, chipped, dusty or stained material). 

In jargon, we call this “fixing the background”. In this way, we avoid making visible the differences in absorption of the substrate, such as plaster or coatings that can leave unaesthetic traces, making the substrate less absorbent or porous.

Consequently, this allows savings, since the substrate will absorb less paint. 

An undercoat or primer also makes it possible to make the substrate more uniform (if it was already coated with an old color), or to apply acrylic paint over old solvent-based paint and avoid the appearance of rust on iron or tannins on wood (chemical compounds naturally present in wood, soluble in water or in certain solvents, colored and which can sometimes produce stains or haloes).

What are the different types of primer?

As with finishing paints, primers (or undercoats) fall into three categories:

Vinyl primer

Vinyl paints are those in which water is used as a diluent, with few pigments and are simply used to prime a new plaster or plasterboard substrate to make it less fragile and porous.

Water-based or acrylic primer

As in the case of vinyl primers, the diluent is water, but these are high-performance paints. Non-flammable, they are divided into matte, satin or glossy, and are suitable for both preparation and finishing, on walls, ceilings, or coffered ceilings.

Environmentally friendly, vinyl and acrylic paints dry quickly, are almost odorless, and easier to use (brushes and rollers can be cleaned with water).

Solvent-based or synthetic primers

The thinner used here is solvent, turpentine, or White Spirit. Composed of synthetic resins, they can be applied directly on all types of paints, raw wood, or wallpapers. Very resistant, they guarantee a good presentation and a nice finish. 

As a primer or undercoat, their high hiding power and consistency allow an excellent finish, especially on stained or damp walls. 

Disadvantages? Its drying time is quite long, its application leaves a strong odor, and cleaning the rollers, brushes (and hands) requires turpentine.

Universal primer 

Compatible with all types of substrates, makes the substrate uniform, hides slight imperfections, promotes adhesion of the finishing paint, facilitates the application and removal of wall coverings, can be applied in all rooms (both dry and damp), and can be covered with all types of decorative paints

Recommended for interior substrates in good condition. Economical for large jobs, it offers good adhesion on plaster, plasterboard, coatings, wood, and cement.

Primer for plasterboard, gypsum board, tempera, or plaster

Microporous (allows air to pass through and transpire on the substrate), it adheres well to substrates such as plasterboard and gypsum boards, coatings and plaster by-products, wood, fabrics or fiberglass. 

This undercoat facilitates the installation and removal of wall coverings, hides the edges of the boards and blocks the very porous bottom of the plaster to make it less fragile and absorbent. 

It is mainly used on new porous or already painted substrates, as it has a better opacity (covering capacity) than the first universal coat. 

Other “special” undercoats for plaster or cement allow to hide possible stains or imperfections and guarantee a homogeneous finish.

Multi-surface primer

With adhesive and insulating properties, it neutralizes stains (nicotine, tannins, humidity) by blocking them on the surface of the substrate. 

Compatible with a multitude of substrates (smooth surfaces, wood, radiators, plaster, tiles, etc.), it can also be coated with acrylic or solvent-based paints. In short, it guarantees good adhesion on all classic substrates, both for indoor and outdoor use.

What are the specific types of primers

Although most of the so-called “universal” primers can cover certain materials such as tiles or plastic, other much more special materials such as PVC, laminate, iron or wrought iron require a specific primer to ensure good adhesion of the paint to the surface.

Primer for walls or decorative coatings

Ideal for waterproofing the walls of a kitchen, bathroom or shower room before laying tiles on plaster or plasterboard; it is also used to improve the direct adhesion of heavy decorative coatings such as wood, plaster, facade plaster, heavy vinyl wallpaper, etc.

Floor, glass or laminate primer

Easy to coat with all types of paints (solvent-based or acrylic), it has a high hiding power and increases the adhesion of the finishing paint on very smooth substrates, such as tiles, glass or laminate floor coverings.

Sealing primer for interior wood

Microporous, it allows the substrate to breathe and promotes the adhesion of the finishing paint. Intended for wood surfaces (doors, windows, skirting boards), it is used exclusively indoors and for any room (dry or damp).

Primer for plastic and PVC 

Reinforces the adhesion of the finishing paint on plasticized or PVC substrates, and resists well outdoors. Can be coated with all types of paints (solvent or acrylic).

Primer for wet surfaces

Hardener and insulator, it regulates the absorption of the funds, it is applied on new and difficult supports, or porous, and in rooms considered “humid” (bathroom, kitchen), or that present a punctual humidity (a laundry room with dryer, for example). 

It is used to clean the wall before painting and provides good resistance to mildew or condensation. Microporous, it has a good adhesion on plaster, plasterboard, wood or fiberglass.

Primer for ferrous metals 

Easy to coat, it guarantees effective protection against rust on ferrous metals and reinforces the adhesion of the finishing paint. It can be painted over with any type of decorative paint (we recommend using a specific finishing paint for exterior use).

Primer for aluminum, copper, zinc, and galvanized 

Resists chemical agents and guarantees the adhesion of the finishing paint on galvanized, aluminum, zinc or copper substrates. All types of decorative paints (solvent or acrylic) can be used over it.

Fixing primer

There are several ranges of primers, the prices of which can vary or even double for the same quantity of product. Here are the criteria that explain these differences.

Covering power

It is the ability of a paint to provide an opaque and covering film, also called coating, capable of hiding by its opacity the shades or color contrasts of the surface. To simplify, the higher the hiding power of an undercoat, the more uniform the result will be.

Filling power 

This refers to the ability of paint to smooth out small irregularities. Of course, this ability is limited and it is essential to prepare the walls (coatings) before applying the primer used on sound, dry and hard substrates.

Yield

It represents the surface you can cover with one liter of product. It is an essential criterion, as it allows you to evaluate the amount of paint you will need. As with finishing paints, the higher this efficiency, the greater the paint savings in terms of quantity and price.

Tips on how to choose

Certain paints already contain specific priming agents for direct application to the surface.

If you are going to carry out small jobs (not very large surfaces), check if you are satisfied with the colors available (some brands have a rather restricted chromatic offer: aluminum gray or concrete, red, yellow, beige, black and white).

If you are going to paint a larger surface (tiles, furniture, etc.), it is often more economical to apply the specific primer on the substrate and then coat it with a classic finishing paint, so you can choose its color, its appearance, and its nature (solvent-based or acrylic).

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