What Is Paint Primer? (& When To Use It?)

Primer is a preparatory coating applied to surfaces before painting. Its purpose is to cover (seal) imperfections, cracks, and holes.

Applying a primer is necessary if the surface is riddled with imperfections, cracks, or holes, or when switching the color shade or the sheen of the finish.

Applying a primer isn’t necessary when the surface is perfect and has no imperfections, or if you are using the same color shade as the existing finish.

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What Does Paint Primer Do?

Primers help to improve adhesion, seal and block stains, prevent moisture, and guarantee a smooth result. The exact primer purpose also depends on its type.

Some primer types, such as shellac-based primer, will seal and block stains and moisture, while a normal primer type won’t. Concrete primer will soak into concrete and cover (fill) holes and cracks. Self-etching primer will corrode the metal surface and increase the adhesion of paint.

All types of primers have the following features.

Improved Adhesion. Primers help to improve the adhesion between the material and paint. The undercoat has extra additives that increase its adhesive qualities and will stick on surfaces that paint won’t. You must apply two (2) coats of primer on slick surfaces, such as plastic, to help the paint stick.
Sealing. Primers seal (or cover) holes, cracks, and dents in the surface and prevent paint wastage. Since the holes and cracks are sealed, they won’t over-absorb paint.
Durability. Primers increase the durability of the finish since they help the paint stick better on the surface. Since paint penetrates and sticks properly, it’s harder for it to peel off — this leads to increased durability.
Smooth Finish. Primers help the paint to produce a smooth finish by providing a smooth undercoat. Since the undercoat is smooth, the paint finish will be also smooth.
Switching Colors. Primers help to switch the color shade of the finish by preventing the old finish from bleeding through. This allows you to apply fewer coats of paint when changing the color shade of the finish.

Is Applying Primer Necessary?

Applying primer is necessary if the surface is riddled with imperfections, cracks, or holes, or is wet, damaged, or non-porous. You can apply paint without a primer, but the adhesion will be weaker, the paint can peel off, the finish won’t be even, the old existing finish can bleed through, and the finish won’t be smooth.

The purpose of primer is to cover (seal) imperfections, cracks, and holes, and prevent water (or moisture) from bleeding through.

A primer coating is necessary for non-porous surfaces as they don’t allow paint to penetrate their surface or stick. But, since primer has extra additives that help it stick better, it will stick over non-porous surfaces and produce a smooth undercoat for the paint to stick over it.

You don’t need to apply a primer coating if the surface is clean, smooth, has no imperfections, and is porous. An undercoat isn’t necessary if the surface has an existing finish (not sealed).

When Should You Apply a Primer?

You should apply a primer in the following scenarios.

When Switching Color Shades. Apply primer when switching the color shade of the finish from dark shade to white shade (or vice versa). The undercoat will prevent the dark color shade from bleeding through the light color shade and staining the finish. A primer allows to change the color shade with fewer paint coatings.
When Switching Paint Sheen. Apply primer between coats when changing the paint sheen (from high-gloss to semi-gloss, or vice versa).
On Fresh Material. Apply a primer on fresh materials (with no existing finish), such as wood, concrete, plaster, or drywall, as they are riddled with imperfections, patches, or moisture.
On Stained Surfaces. Apply stain-blocking primer when painting over wet, stained, filthy, or greasy surfaces.
When Painting Wood Trim. Apply primer when painting wood trims to help the finish last longer.
When Painting Slick Surfaces. Apply primer when painting slick surfaces to help the paint stick. If you don’t. the paint won’t stick over slick (non-porous) surfaces.
On Porous Surface. Apply primer if you are painting porous surfaces, such as wood, plaster, drywall, to prevent over-absorption and help paint stick better.
On Uneven Surfaces. Apply primer on a rough or uneven surface if you want to get an even or smooth finish.

When Shouldn’t You Apply a Primer?

You shouldn’t apply a primer in the following scenarios.

If The Surface is Perfect. Don’t apply a primer if the surface is smooth, dry, clean, even, and has no imperfections as there’s nothing preventing the paint from sticking or producing a smooth finish.
If Using The Same Color Shade. Don’t apply a primer if the surface has an existing finish (unsealed) with the same color shade. In such cases, light sanding with fine-grit sandpaper is all you need before applying paint.
Old Material. Don’t apply a primer if you are painting over an old material (surface) and don’t need a smooth finish.

What Happens if You Paint Without Applying Primer?

If you paint without applying primer, the following things will happen.

1. Paint Adhesion Will Be Weak

The paint adhesion will be weak if you don’t apply a primer coating. Primer improves the adhesion between the paint and the material (surface). When you apply it, the paint sticks over it and not the surface. This improves the adhesion of the paint.

But, if you don’t apply a base coat, the adhesion between the paint and the surface will be weak. If you apply paint over a non-porous surface without one, the paint won’t stick at all.

2. The Finish Will Be Rough

The finish will be uneven and rough if you paint without a primer because the imperfections on the surface won’t be sealed (or covered) so they will show once the paint dries.

However, if you apply a primer, it will cover or seal these imperfections and provide a smooth surface for the paint to stick to.

3. Moisture Will Bleed-Through

The moisture in the surface will bleed through on the finish if you don’t apply a primer first. Stain-blocking primers will cover and prevent moisture from affecting the finish.

If you don’t prime a wet surface before painting, the moisture and stains can easily bleed through the paint causing the finish to appear stained and blotted.

4. Cleaning Will Be Difficult

Without a primer, the paint won’t adhere properly to a surface. If you clean a paint coating that isn’t bonded perfectly to the surface, the paint can peel off while cleaning due to weak adhesion.

5. Old Paint Can Bleed-through

If you paint over old paint without applying a base coat, the old coating color shade can bleed through the new coating. For instance, if you apply light-colored paint over dark-colored paint, the old dark-colored paint can affect the light-colored paint. This is called bleed-through.

However, a primer coating will prevent the old paint from bleeding through.

Do You Need to Apply Primer Before Painting Wood?

You need to apply primer before painting wood because wood is a porous material that can suck too much paint, and lead to paint wastage. The primer will cover the wood surface and prevent over-absorption or paint wastage.

Wood is usually riddled with imperfections that can affect the finish qualities. So, before painting wood, apply a stain-blocking primer to allow the paint to stick properly.

However, a primer coating will cover (hide) the wood grain color. So, if you want to apply a clear coat for the wood grain to show, don’t apply it. Also, you shouldn’t apply it to wood if you want to apply wood oil or sealant.