Wood is the most common surface for paint primers, a fact that is hardly surprising considering its organic nature. The reason you always want to prime wood before painting is twofold:

• To protect the paint

• To protect the wood

A number of modern paints dry into proper color and hardness via evaporation, meaning they require the water to find its way into the atmosphere. The problem is that wood is a notoriously thirsty substance, and it is easy for water to get absorbed into the grain itself. Often the paint will begin to pucker and peel if the wood isn’t dry, making it necessary to scrape and start over. Paint primer creates a watertight seal between the two and ensures the paint has a smooth and adhesive surface with which to bond.

The second reason you always want to prime a wood surface is because the same absorption can quickly undo your attempts at an even coat. Lighter paint colors especially will often reveal imperfections in wood grain even after several passes, requiring you to paint over and over for a truly solid hue. Although you can apply paint repeatedly to achieve the same effect, a fast coat of primer first will save you time and money.  

The wood itself can be injured without a primer in place. Although a negligible amount of water is absorbed during drying, greater amounts may leech into the grain if that painted surface is exposed to the elements. Without the use of a primer it’s not unusual for repeated rain storms to take their toll, creating devastating conditions for mold, mildew and warping. Although many people consider outdoor paints to be waterproof, often they are not because their hardy nature derives from their ability to absorb rainwater with ease.

Paint primer is recommended for a variety of other materials too. In concrete and drywall, for instance, you simply want to seal the surface and create a solid bond without having to worry about similar problems with drying and moisture.

For metals, especially those prone to rust, a paint primer can protect the surface itself and keep moisture locked out avoiding rust and pealing. Porous plastics can benefit from a coat of primer or if you are putting a light color over a darker color.