Primers are designed to get a surface ready for the next stage of decoration. It's important to know how many layers of primer you should apply before you paint. Too few layers can result in the wall not being properly prepared, while too many layers is a waste of time and money. There are different types, which influence how many layers are needed.
An oil primer acts as a stain blocker. If there are stubborn stains on the surface that you cannot remove, a single layer of latex primer should cover the stain. A maximum of one more layer can be used if one layer doesn't cover it.
If painting a home where there are smokers and the walls are stained from cigarette smoke, latex primer will not cover the stain, an oil-based primer or sealer will be required instead.
Latex primer has a similar consistency to oil primer although it will be slightly thinner. Latex primer is ideal for use on walls, which are subjected to dampness. You first have to dry up the dampness on the wall. Fans will work. Oil and water don’t mix, so a damp wall will most likely not receive an oil-based primer. Being water-based, latex primer will be absorbed by the damp walls.
No more than two layers should be used on a damp wall before another paint is applied. A single layer will be enough if the wall is not damaged in any way. A thin, single layer should be applied to a wall where a specific decoration or area of paint will go.
You can also get masonry primer, which should only be used on masonry blocks or surfaces, and requires a single layer only.
Tinted Shellac Primer
Tinted shellac primer has a specific purpose, masking severely damaged walls. These may be walls damaged by dampness or smoke. For smoke damage from a fire or heater blowback, shellac or alcohol primer is the first choice. A single layer is enough to hide minor problems. However, further layers should be added until smoke or water damage has disappeared.
The primer will not affect the color of the finish paint. You can also have the primer tinted to the chosen finish color, and this will generally eliminate one coat of paint. Two layers of application should be enough. Tinted shellac primer is very good at masking smells. Layers can be added to mask the stale smell of dampness or an area where a pet may have urinated.
There is another type of shellac called orange shellac, which is the only thing that can keep knots in wood from eventually showing through the finish coat.
Clear Coat Primer
Clearcoat primer is perfect for using on newer walls. This primer is much thinner than other alternatives. The purpose of a clear coat primer is to create a surface over a wall that will help the paint to stick. If you're planning to paint a brand new wall, a single layer is all that will be needed. If the wall isn't brand new but is in very good condition and undamaged, apply up to three layers before you start your painting job.
Caution: It is very important that you always wear protective gear including a respirator when using oil- and shellac-based primers.