The Difference Between Primer, Sealer and Undercoat Paint

Painting a wall might seem simple enough at a glance, but there is a lot of preparing that you have to do before you apply that first coat of paint. Namely, you’ll have to figure out if you’ll need to use an undercoat on the surface that you’re going to paint over, and if you need a primer or a sealer paint.

Those three paints will define how the painted surface will look once the paint dries. So, what is the difference between a primer, a sealer and an undercoat paint? We’ve broken things up for you to understand, even if you aren’t an experienced painter.

What is primer paint?

Primer paint is applied directly to the surface that you’re going to paint over. The main function of the coat of primer paint is to give the top layer good adhesion to the substrate. Other functions of primer paint include:

How to prepare a surface for primer paint?

In order for the primer to work best, the surface should be properly prepared before priming.

1. Wash the substrate surface.
   Ideally to be done 24 hours before applying the first coat of primer paint, use a damp cloth to wash the walls and remove any accumulated dust on the walls.
2. Fill any holes and cracks with spackle.
   Make sure to fill any cracks and holes in the surface with spackle. Do this a couple of hours before applying the initial coat of primer paint.
3. Sand any glossy surfaces.
   Glossy paints and surfaces are harder to paint over. Sand them in order to roughen the surface and to create a better bonding substrate.
4. Dust the substrate with a dry cloth.
   When you’re sanding the glossy surface and the excess spackle you’ll create a lot of dust. It’s important to remove any dust in order to maximise the bond between the substrate and the primer paint.

What is sealer paint?

Sealer paint can be used as a substitute for a primer, or before the primer is applied to the base surface. The main purpose of sealer paint is to provide good adhesion between the substrate and the new paint or the primer.

One of the biggest pluses of sealers is that they can recondition old surface unfit for painting. If you’re going to paint over a porous substrate, we advise you to use a sealer if you want to make your painting job a hassle-free experience.

Other uses of sealers can be to seal and retain any waxes in the substrate and protect the painted surface from staining.

What is undercoat paint?

Undercoat paint is applied before the initial layer of paint. It’s supposed to further improve the bond between the primer/sealer and the top layer of paint.

Simply said, undercoat paint is used by handyman professionals to further strengthen and help the functions that primers and sealers do. These include moisture-proofing the substrate, even out small imperfections and improving the bond between the top coat and the primer or sealer.