There are lots of primers available from a lot of different manufacturers that do a lot of different things. It can be quite confusing as to what they are all for. At the end of the day though, generally speaking, primers only do three main things: One, block things from coming through your paint job. Two, act as a tie or bonding coat to get a coating to adhere to a surface. Three, seal substrates to provide a uniform foundation for the finish coats. Some are multifunctional and can do two or three of these things at one time, while others are highly specialized and only do one thing really well.
Blocking Primers – Most people have heard of these primers and use them to block water stains, smoke stains, odors (pet urine) and rust. They go by names like Kilz, Cover Stain, Extreme Block and Rust Destroyer. Typically, the stronger the smell of these primers and the more difficult they are to use, the better they are at doing their job. For example, Cover Stain Oil Base Primer by Zinsser is a high-quality, stain blocking primer that blocks many different types of stains, including smoke and chemical odors, but it has a strong smell and is harder to work with because it’s oil based. Kilz 2 Latex is a Water Base Stain Blocking Primer that is low odor and much easier to work with, but won’t block stains as well and may allow them to come through your freshly painted surface. Not sure how strong a stain blocking primer you may need? A quick call to your local, professional paint store should get you what you need quickly and with authority. Or, you can do your own research online and read through the different primers out there and what they offer.
Tie Coat or Bonding Primers – While most paints will stick to a lot of different surfaces, how well and how long they will stick to that surface depends on whether or not you use a bonding primer. If you have ever seen paint peeling off a door or other piece of trim in ribbons, then you have seen an example of this. The paint sticks initially, but normal use and wear and tear causes the paint to peel quite easily. If you can peel paint off a surface with your fingernail, you can be sure a bonding primer was not used first. For example, if you have an older home that still has old, oil base paint on the trim, you will need a bonding primer to act as a tie coat for the new, water base paint that we use today. We all know oil and water don’t mix. The bonding primer is designed to adhere to the oil base surface and the water based paint at the same time. Which is why a bonding primer is sometimes called a tie coat, as it “ties” two coats together. Other surfaces that will require a bonding primer are things like prefinished metal (gutters, flashing, metal roofs, roll up garage doors), tile, plastics or other extremely hard or very smooth surfaces. Typically, the more expensive the primer, the better adhesion it will give you. A good, general purpose bonding primer, like Zinsser Bulls Eye 1-2-3, can sell for between $15 and $20 a gallon and can handle basic bonding primer situations. Or, you can spend $50+ a gallon for X-I-M’s (Rust-Oleum) UMA (Urethane Modified Acrylic) bonding primer for projects that will require more adhesion in tougher situations. If you wanted to paint a prefinished, metal roof to a new color, spend the extra money and go with the X-I-M UMA bonding primer.
Sealers or Undercoaters – These primers are used to ensure that the finish paint looks and feels like it should for the surface being painted. For example, drywall that has been taped, mudded, textured and sanded will need a primer prior to finish painting. The end result of all of that work to get the look that you want for your drywall is a surface that varies tremendously in porosity. If you paint over this without the correct primer that seals the porous surface, you will end up with a splotchy and uneven finished product. PVA (Polyvinyl Acetate) primer is an inexpensive primer specifically designed to penetrate and seal drywall surfaces. It allows for finish paint to penetrate evenly and produce a uniform finish. The same principle applies to fine finishes on wood surfaces. Think cabinet painting or wood trim painting. Wood also has different porosity. Primers for wood are typically called undercoaters. They can be water based, oil based, alcohol based (shellac primer) or lacquer based. They will all seal the wood to achieve the consistency in the finish you need, but the oil, alcohol and lacquer based ones will sand easier and produce a smoother, finer finished product. Cement based surfaces, like stucco, CMU block and concrete tilt up walls will also benefit from a primer for the same reason, they are porous surfaces that will look blotchy and uneven without a primer.
While you may not want to spend the extra time and money on a primer coat, do yourself a favor and do so. You will end up with a nicer looking finished product that will last longer.