Using Primers Before You Paint

If you fancy giving a room a makeover, painting the walls is one of the quickest and most effective ways of injecting a new and refreshing aesthetic. However, if you don't have the required know-how, the straightforward task of painting your walls could become a mistake-ridden nightmare and, as we all know, accidents cost money, time and effort to rectify. Therefore, it is wise to know when it is worth using a primer before you paint your walls, and when this is not necessary.
In rooms such as the kitchen and bathroom where a high level of grease or oil residue will undoubtedly have saturated into the walls over time, it is important that you clean the area first. Simply wash the walls with a mixture of ammonia with warm water, which will allow your new paint to bond to the walls much better. Some people like to use Trisodium Phosphate (TSP) to strip off any gloss that is present on the walls due to the previous homeowners, although this is not necessary unless the area has been covered with high-gloss or semi-gloss. You can also purchase commercially available Wall and Ceiling Cleaner products to get the job done.
Painting Walls a Light Color

If you want to paint your walls a cream, gray, white or light pastel color, such as baby blue or light green, you will not need a primer. The general rule of thumb is you do not have to use a primer to go from a lighter color to a dark one, although you may sometimes need to paint the surface with more than one coat.

A benefit to priming the walls beforehand is that it can help to block old stains like rust, crayon or marker pen, or even more stubborn wall discolorations due to mildew, nicotine smoke or water stains. After application, another advantage priming offers is that it will facilitate a longer-lasting paint job, since primers prevent paint from peeling, chipping, cracking, chalking, flaking and blistering. This means that your paint job will look fresher and newer for longer, as well as retaining its basic color tone. Primers can also help if you want to paint onto certain problem surfaces, including ceramic tiling and Formica cabinets.
If the old paint covering your walls is much darker and aggressive than the new color you want, it is worth using a tinted primer and then adding your paint afterward. By doing this, you will not be forced to use several coats of paint to "drown out" the darker color found beneath. Bear in mind that going from a light or pale color to a stronger, darker shade means that you will need the walls to be primed first, such as a new cream color being painted over a dark red or deep blue wall.
If In Doubt, Take The Quick Tape Test!

This handy test will determine whether or not the already-painted area will offer a good surface to apply your desired paint color of choice. Grab a roll of tape and peel a small piece off. Attach the sticky side onto the surface you are testing and then suddenly rip it away, really wrench it off quite aggressively. If there is a powdery substance left on the sticky side of the tape, the surface should be thoroughly washed. But if there are any paint flake remnants stuck to the sticky side, the old paint needs to be sanded down.
When it comes to painting the exterior of a house instead of the rooms inside, you do not need to use a primer if the paint is still in good condition. However, due to drastic changes in temperature and weather throughout the year, that's typically a rarity. It's generally a shrewd idea to spot-prime any bare wood with a high-quality acrylic/latex exterior primer if the paint has already cracked and peeled away. Before doing so, meticulously scrub the area and then rinse away any detritus with a power washer. Pigment-rich woods such as redwood and cedar contain tannins that have the power to eventually bleed through lighter colors as well as standard primers, so you will have to use a specialized oil-based stain-blocking exterior primer instead.
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