The word “primer” comes from a Latin word that means “first”. Among painters, it means a preparatory coating that is first painted on a surface before the finishing or final paint.
Paint primers are special type of paints that perform important functions. They are physically and chemically engineered to be very sticky holding fast to the surface they are painted on. Apart from this, they also bind extremely well with finish paints applied over them. As far as this function is concerned, you can think of paint primers as the glue that binds the surface of your work with the final paint.
In addition to their adhesive functions, primers also seal and smooth porous surfaces such as concrete and wood. Sealing porous surfaces is important because they tend to absorb the finishing paint’s solvent too quickly. This adversely affects the curing process of the paint leading to unsatisfactory results. With primers preventing such absorption, the curing process can be completed as required.
Wood, depending on its species, can have differently hued grain bands with springwood having lighter tones while summerwood is darker. The former is also more porous and softer. When painting such wood, primers are needed to even out their differences in color and porosity. Painting without primers will result in a dull finish over the springwood and a glossy appearance on the summerwood.
Primers also help the coated surface resist moisture. They are therefore especially helpful when painting exterior surfaces that are exposed to the elements. While finish paints are waterproof to a certain extent, primers further reinforce this. Working together, they can prevent wood from rotting and warping, and metal from rusting and corroding.
When painting over old paint with stains, cracks, and chips, primers do a good job of concealing and filling them up. If the new color is lighter than the old paint, primers can be used to cover the dark color so that it will not be visible through the lighter paint. You can even tint the primer with the color of the finish paint to reduce the number of coatings needed to bring out the final color.
Different surfaces, of course, require different primers. Be sure to explain to your supplier the type of surface you want to prime, whether it is outdoors or indoors, and what type of paint you will eventually apply over them.