Primer:  While technically not paint or a finish, the use of primer is crucial in interior painting. Failing to use primer on a porous surface, such as wood or concrete, can lead to imperfect paint coatings that could end up peeling away from the walls. Primer is also a must if covering up a darker color, as it will take fewer layers to cover the old paint.
Primers aren't just diluted paint. They're formulated to establish a solid, even base, seal stains and ensure that the topcoats of paint go on smoothly and bond securely to the surface. "Most homeowners use latex primers, but the pros stick to alcohol and alkyd primers because they'll cover almost anything," says John Weeks, of John the Painter in Mobile, Alabama. Primer can affect the appearance of the topcoat. "It's okay to spot-prime the ceilings but not the walls, because primed spots will show," adds Span.
The right paint stroke to use in interior painting is highly debatable. It's not a talent as much as a skill that is learned through practice. Many experts paint in a “W” pattern when using foam rollers, but others simply roll up and down then sideways with either brushes or rollers. The motion isn’t as important as making sure that the application is even and drip-free across the entire area. However, technique is important when you're covering over wall repairs.

Home Painter Co