Primers and Paints
Primers and finish paints have different functions in the decorating process, and it's critical to understand the role each plays. Finish paints provide surfaces with a final, smooth layer of protection and a final color.
Primers are used to prepare those surfaces for finish paints and aren't suitable for finish coats.
Why Use Primers?
Without priming, new wood, plaster or drywall surfaces will soak up more finish paint in some areas than in others, producing a splotchy effect, and you end up using more paint to even out the finish, which is not economical because it's more expensive than primer.
In addition to sealing the surfaces, good primers will also cover different textures, like the difference between drywall and joint compound. And that's critical if you're using higher-gloss paints which tend to show those differences and imperfections more than flat paints.
A third benefit of using a primer is that it provides the finish paint a better bonding surface so it sticks to the base material much better.
Latex primers generally work best over drywall, plaster and concrete surfaces. They dry fairly quickly (usually about an hour) and are water soluble so they can be cleaned up with soap and water.
Alkyd primers generally work best on raw wood, they take a fairly long time to dry (usually overnight) and require mineral spirits or paint thinner for clean-up.
Stain-killing primer-sealers (often alcohol-based) are specially formulated to prime and seal problem areas like water stains on walls or knots in bare wood which can bleed through ordinary primers and show through the finish paint. They come in several formats: water-based, oil-based and shellac-based (which cleans up with alcohol). They usually cost a few dollars more per gallon, but are often worth the price for better coverage over trouble spots.
Finish paints are categorized by the amount of surface shine they produce when dry. Terms like "gloss" and "flat" often dictate where such paints are used in the home but their use does overlap in some areas.
High-Gloss Paints, often referred to as "enamels," are the shiniest and most reflective paints. They produce the hardest, most water-resistant and most washable surface coating, so they're most often used on wood trim and kitchen, bath and playroom walls. However, surface flaws show most when covered with high-gloss paint.
Semi-Gloss Paints, also known as "eggshell," "velvet," or "satin" paints, produce a somewhat shiny surface that's more reflective than a flat paint but less reflective than a high-gloss surface. They resist moisture better than a flat paint, so they're more washable and serve well on walls in hallways, kitchens, baths, and children's bedrooms.
Flat Paints leave a "dull" or "matte" finish, with no gloss, shine or reflectivity. They work best on irregular wall surfaces where you want to hide the imperfections as much as possible. Flat most typically used on ceilings, living rooms, and dining rooms over drywall surfaces.
Finish paints, like primers, also come in both latex and alkyd-based formulations.
In general, the latex products are best suited for drywall and the alkyd-based paints work best on wood trim.
Although latex formulations are improving all the time, alkyd-based high-gloss paints usually leave a smoother finish since they take longer to dry and have more time to level out brush marks which sometimes remain visible in latex paints.