Maybe you’ve been to the paint store and heard a salesman say, “You have to use the right primer, or the paint won’t adhere to the surface properly.” and wondered how that was possible. Well, they aren’t just trying to earn a commission, you have to use both paint and primer for specific reasons.
First of all, paint and primer are two different things. You can’t use one in place of the other. Although you can paint drywall or plywood without priming the end result is not as nice or as long lasting. The paint is likely to peal off the surface in a matter of months, unless you use a high quality, exterior paint and primer combination.
All materials, whether they are solid wood, or solid metal, have a pitted surface. Primer is applied to cover up this rough surface. Paint sticks better to surfaces that are smooth, and very consistent. Even after sanding an object, grooves still exist, and several coats of good primer will cover them up.
Special primers chemically bond with paint for a really great finish, however the primer itself has to bond with the surface material first in order for the paint job to last especially long. Make sure the primer is made for the job, in other words. Buy wood paint or drywall paint if that is what you intend to paint.
You still need to sand in order to prime and paint something, the sanding just makes the surface that much smoother and consistent. It also provides a conditioned surface the primer can bond to.
Using several coats of primer is recommended over really rough surfaces or drywall. Even though drywall feels smooth to the touch, it has a consistent series of very small ridges and grooves due to the texture of the paper used to create the material. Paint and primer is an absolute must when finishing your home.