You may have taken on a painting job yourself in the past, and been dissatisfied when the wall you painted appears to have all kinds of bumps and imperfections. This is probably because you didn’t take the time to completely sand away any flaws on the surface you had in mind. Having a perfectly smooth surface is integral to the smooth, flawless end result you’re looking for. A thorough sanding job gets rid of joint-compound patches and spackle, and levels out the ridges left around nail and screw holes, thus ultimately leading to a much more even- and good-looking paint job.
Another mistake people often make is failing to use a tinted primer. Before professional painters such as CertaPro Painters of Waterloo apply a single drop of paint, they spend a lot of time filling in holes and levelling out cracks with joint compound.