The Difference. Before choosing to apply a primer, sealer or undercoat, you will need to learn the difference. 

The Age. If you are about to paint a new surface, use a primer. If you are considering painting an old surface, use an undercoat

The Surface. Different surfaces require different primers or sealers. You will need to read the instructions carefully in order to choose the right primer for the surface.

The Type. There are different types of primers, sealers and undercoats available for your use. The right type will depend on the surface that needs to be primed. The main types are oil, shellac and latex.

Application. When the right product is chosen, you will need to decide how to apply it. You can choose between the roll-on and spray option depending on your needs and the kind of object you are painting.

The Budget. There are several things you can save money on when choosing one of the three products. Knowing which types fit your goal best will help you operate with a low budget.

Sealers are applied to the surface before the painting process starts. Their main goal is to seal all the imperfections that appear on the substrate. They are made out of special ingredients to make the sealing fast and productive. Sealers are also used as a barrier between different finishing coats. They are especially useful for overcoating old finishes and making porous surfaces non-porous.

Primers are the first coat that is a applied to the surface before painting. The main purposes of a primer is to penetrate the surface and make it uniform to allow smooth painting. It can also benefit the adhesion of the coating system. Good primers provide different protection (including corrosion) to the surface.

Undercoats are used to smooth out the surface in order to allow the paint to spread evenly and provide and even sheen. Some manufacturers offer 3-in-1 products that act as a sealer, primer and undercoat at once.

Wood. Wooden surfaces need to be filled and leveled so the paint doesn't soak through. It is recommended to use a special wood sealer for the such surfaces.

Metal. Metal surfaces require a special non-corrosive primer that will assist the paint in bonding to the metal. There are also primers that can prevent rust. They can even be applied directly onto rusty surfaces.

MDF. MDF is porous and absorbs everything that is painted onto it. That's why it requires a special MDF sealer that won't allow the paint to soak through.

Plaster.  Plaster is a porous material. It will require a special plaster sealer to prevent the paint from soaking through. If special plaster sealer is not used, more coats of paint will be required.

Tiles. A special tile primer should be used to help the paint attach evenly to this hard and smooth surface.

Multiple surfaces. If you need to paint many different small surfaces, it would be a good idea to buy the 3-in-1 primer/sealer/undercoat product. It will save you money on buying them all separately.  3-in-1 products can be used on all surfaces. However, it is still preferable to use specialized primers/sealers/undercoats best suitable for each surface.

Oil primer/sealer/undercoat. These are slowly-drying products that can produce a very smooth surface. They do a good job filling the pores in raw wood. They also offer a good barrier and don't allow the paint to soak through and the tannins to leek out. Such primers are perfect for all types of wood and can be used over already painted surfaces in case the paint is cracking.

Latex primer/sealer/undercoat. These products dry very fast and are water-soluble. They can be found in low-VOC formulas or can contain no VOC at all.  They offer a better flexible finish than their  oil and shellac counterparts and are less brittle. They make a good first coating for raw softwoods. Latex undercoats are the best option for drywalls. They do a good job evening out the surfaces. They let the water evaporate which allows them to last longer.  Latex primers can also be used for masonry and galvanized metal.

Shellac primers/sealers/undercoats. These products are fast-drying. They use denatured alcohol. However, they provide a strong and unpleasant odor and can be difficult to use. They are perfect for stain blockage and preventing tannin bleeding. These products are also good for water and rust stains and can be applied to different surfaces whenever you need the primer to dry fast.

Stains and odors.  If you are faced with stains that show through the surface regardless of the amount of primer or the number of coats you apply or need to fight the cigarette smoke that stays locked in the substrate, you need a stain-blocking primer.  You can purchase either an oil-based or water-based primer.  Choose oil-based primer if you need to get rid of rust, smoke, wood tannins and water.  Get water-based product to make cleaning easy and avoid unpleasant odor. They are perfect for solvent-based stains such as grease, ink and crayons. Stain-blocking primers are white and will need to be tinted if you are planning to use dark paint.

High-moisture areas. If you are looking for the best products for the areas that are high in moisture and prone to mildew such as bathrooms, kitchens and laundry rooms, you need a vapor barrier sealer.  Humidity is one of the worst paint enemies since it causes peeling, flaking and blistering.  Vapor barrier seal will seal the surface and will not allow most of the moisture to go through. If you are also faced with mildew, you'll need a stain-blocking primer followed by a special mildew-reducing topcoat (it will be marked as such). All existing mildew must be removed before painting.

Previously painted wood. If you are faced with previously painted interior wood such as doors and the paint is chipping or flaking, you will need to purchase an oil-based undercoat. It will do a perfect job attaching to the previously painted wooden surfaces and staying smooth in the process. Fast drying primers and sealers will soon become brittle. Oil-based primers will take about 2 days to dry but will produce a good result. If you still wish to opt for a water-based primer, look for acrylic-latex undercoat that can be sanded.

Color switch.  If you need to follow through with a drastic color change (from very light to very dark or the other way around), you will need too many paint coats to do it. Choose a primer that can be tinted with lighter and darker colors in order to reduce the number of coats that will need to be applied.  Tinting a primer gray helps the color become richer and help hide surface imperfections.

Exterior surfaces. Exterior surfaces always need special care since they are exposed to different weather conditions, including extreme temperatures and precipitation. The paint on the exterior surfaces will last longer if a good primer is used. The best choice will be the highest quality acrylic-latex primer for exterior surfaces. The label should read “100% acrylic.” If the primer is well-chosen you can extend the life of paint by several years.

Drywall (new or patched). The products that are used for the drywall seams react differently to the paint than the rest of the surface does. The difference between the surfaces can lead to flashing and improper sheen. In order to paint the drywall, you can use a drywall sealer. Standard drywall sealer is good for perfectly smooth surfaces. If there are some imperfections, you should go for a high-build drywall sealer which can be more expensive. However, it will do a better job making the surfaces leveled-out.