Paint vs. Primer

Paint vs. Primer: What's the Difference?

Without getting too technical, the basic difference between paint and primer is that of resins versus pigments. The resins contained in primers seal porous surfaces and provide the bond to the surface. This is primer's most basic function, to provide a sealed, bonded, stable surface on which to apply your topcoat. Paints contain pigments, which, in addition to providing color, allow for better hide and give paints the durability to withstand the elements. Each function of a primer, and every characteristic of a finish, requires a different chemical formulation. Given all the various substrates, the problems they naturally incur, and all the elements each is required to protect against, how do all those formulations fit into one universal product? It is in the various substrates and their characteristics where the trouble with "paint and primer in one" lies and in the lack of education regarding these differences.

In addition to their most basic function, primers are also problem solvers. Substrates have different characteristics that require different primer functions. To name a few situations, wood contains knots and tannic acid; metal rusts; and some surfaces make adhesion difficult. Oil-based paints are not self-priming when applied to galvanized metals, because without a latex primer, the oil will peel. The chemistry of galvanization will reject an oil paint. The increasing popularity of synthetic building materials calls for primers specifically designed to bond to smooth, non-porous substrates. Not all primers bond this way, but some paint finishes will. Water, grease, ink, and various other surface stains and contaminants will require specific primers to prevent bleeding through the topcoat. Some substrates will require a combination of primers. For example, if a wood project calls for a latex primer/finish system, the knots will require sealing with a shellac-based primer prior to full latex priming. How could one expect a single product to address all of these conditions? And these are but a few of the most common.

When Is a Primer Recommended?

In addition to the obvious conditions stated above, in a few other instances a primer will improve the outcome of a painting project. One mistaken belief is that a full primer coat is required prior to all repainting work… Not so. Most previously painted surfaces will require a spot priming at most. A full coat may be preferable if the spots are varied and multiple. Some insist on applying an oil primer as an intermediate coat when changing from an oil finish to a latex finish. Others insist on priming when changing from dark to light colors. Some primers are high build, meaning that they are designed to smooth out rough surfaces or fill surfaces, such as concrete block. For old, weathered, or otherwise degraded surfaces, a full prime coat will provide a fresh start for a new finish. For this type of primer application, the primer will seal the degraded surface and not leave the first coat of finish to perform that task. In such a case, the possibility exists that in performing that task, the second finish coat will not develop fully with respect to sheen, color, and uniformity. This may be a task where self-priming paint, or paint and primer in one, may be effective. However, it is not something I rely on personally.

No One-Size-Fits-All Answer

Some finish paints will coincidentally perform a combination of tasks, as implied earlier. However, how can you predict which one will? Do you take a chance? An experienced painting contractor, knowledgeable about substrates, and with an understanding of product capabilities through years of use, will know. Even so, with over two decades of experience, I sometimes have to do some research.

Many consumers mistakenly believe that paint is paint and primer is primer. These consumers may, without some knowledge regarding primer purposes, expect paint and primer in one to provide an inherent solution to any set of circumstances. Such an assumption could result in paint failure and possibly costly repairs. My hope is for my readers to avoid such oversights and approach the issue of primers with a more critical eye toward their needs. In the absence of a knowledgeable paint retailer, detailed product information may not be readily available, but it is worth taking the time to do your research and make an informed decision… and then the results are on you.