Primers (General)

The benefit of priming, and the role of primers, is often misunderstood in the paint and decorating products industry.  The key to achieving the highest level of performance from most top or finish coats can be directly attributed to the level of surface preparation . . . which often includes priming.

Are primers just "thinned down" versions of paint?
     Contrary to some opinions, primers are not just "reduced" versions of the finish coat.  Primers are really uniquely formulated to perform specific functions.  Oftentimes, it is a greater challenge to formulate the primer than it is to formulate the finish coat.  No single primer can perform all functions well, thus numerous primers are needed to meet the varying demands in the paint and decorating products industry.

What are some of the functions that a primer performs?
     We offer the following as a partial list of functions that a primer might be asked to perform:

  • Provide uniform porosity resulting in a smooth, even top coat film and consistent appearance or gloss.
  • Provide a uniform texture or grain for the base.
  • Give "tooth" for adhesion of subsequent coats.
  • Tie down chalky surfaces.
  • Tolerate moist surfaces.
  • Inhibit rusting of iron and steel.
  • Allow adhesion to non-porous substrates (e.g. hard, glossy surfaces such as aluminum, copper or plastic).
  • Act as a barrier coat for top coats (e.g. oil/alkyd over galvanized steel or epoxies over alkyds).
  • Act as a barrier coat for substrates (e.g. alkali resistant or tannin blocking).
  • Perform other stain blocking (i.e. cover fire damage, water spotting, or other bleed-through problems).
  • Contribute to hiding.
  • Provide additional mildew resistance.
  • Provide a vapor barrier.
  • Enable quicker recoat (i.e. quick drying).
What's the difference between undercoaters, sealers, fillers and primers?
     Undercoating is an older term for priming.  The term undercoating was typically used when the primer was applied as a base coat for enamels - such as automotive paints - or when applied to improve the seal and provide a "base" for the topcoat.  Sealers are liquid coats which seal wood, plaster, wallboard, etc. and prevent the surface from absorbing paint or varnish.  Sealers may act as a primer or even a finish coat in the case of clear wood finishing.  Fillers are usually pigmented, heavy-bodied liquids used for filling cracks and indentations in wood, metal, plastic, masonry, etc. to obtain a smooth, even surface before paint is applied.  Primers are defined as the first complete coat of a painting system applied to a surface.

What makes primers differ from undercoaters and sealers?
     Primers are designed to provide adhesion to new surfaces and are formulated to meet the special requirements of many varied surfaces encountered in today's world.  The type of primer varies by the surface, its condition and the total painting or wallcovering system to be used.  Hence, primers for new wood and certain other surfaces must provide for exceptional absorption to the substrate, good hold-out and facilitate the adhesion of the topcoat.  Primers for iron and steel must also contain special pigments that inhibit or fight rusting and corrosion.  These anti-corrosion pigments often include zinc chromate or zinc powder.  Red lead pigment was frequently used - until recent years, however, lead ingestion problems have resulted in the disappearance of red lead primers from the industrial scene.

When should a primer be used?
     When painting a surface that has never been wallpapered or painted before, a primer should be used.  Examples might include new wood, wallboard, stucco, plaster, etc.  Primers can also solve some difficult problems . . . situations that require more than just a coat of paint.  Rohm & Haas' PQI (Paint Quality Institute) suggests that primers might help:

  • Prevent bleed-through from "staining woods" like cedar or redwood.
  • Cover household stains or graffiti.
  • Hide water or fire damage.
  • Seal porous surfaces like weathered wood or masonry, sheet rock or plaster.
  • Help paint adhere to slick surfaces like tile or high-gloss enamels.
  • Seal off knots and sap streaks in wood.
Primers Interior & Exterior

The best paint performance is a result of good surface preparation and proper priming or sealing.  However, even the pros seek advice when deciding how to prepare the surface and which primer to use. 

Where and when should a primer be used?
     Primers are typically used where a surface has never been painted before - like new drywall, plaster or wood.  They are frequently used when previously painted surfaces are badly deteriorated, uneven, stained, weathered or are very porous.  Note the drawing (courtesy of Wm. Zinsser & Co., Inc.) that points out 17 different areas around a house that might need priming.

Is there one single primer that will work in all applications?
     Not really, that's why there are so many primers on the market.  Many manufacturers do offer a "universal" primer/sealer/stain-killer with broad performance capabilities.  However, the performance properties may not always satisfy the task at hand.  To help understand the differences in primers, it might be simpler to think in terms of interior primers, sealers, and stain covers, exterior primers, sealers and stain covers and primers for different kinds of metals.

What's the difference between an interior primer and an exterior primer?
     Until recent years, most exterior primers were oil based, for maximum penetration into weathered wood surfaces.  Most interior primers were latex for their quick drying properties.  The technology has changed rapidly in the past few years, and the manufacturers have been able to develop excellent formulations for exterior application using waterborne technology.  So, the differences relate more to performance requirements than to latex or oil formulations.  Exterior primers must provide deep penetration, seal out cedar or tannin bleed-through, provide outstanding inter-coat adhesion and good enamel hold-out so the finish coat looks smooth and even.


Image courtesy of Wm. Zinsser & Co.

What are the performance requirements for interior primers and sealers?
     Interior primers must seal surfaces, provide good enamel hold-out and dry quickly.  They must provide a uniform base over drywall, spackled areas, trim, molding, plywood, etc. as well as stained surfaces.  After the finish coat is applied, there should be no visible differences in appearance over the entire painted surface.  This means uniform texture and gloss, the absence of shiny spots (shiners) and uniform color.

Are there special primers for metal objects?
     Most metal surfaces differ in their priming requirements.  Galvanized Steel, for example, should be weathered for a few months to remove "mill scale" and other contaminants before painting.  Then it should be primed with specially formulated waterborne primers.  Painting with oil based primers will result in "major peeling" in a matter of weeks.  The oil-based resin (usually an alkyd) chemically reacts with the galvanizing and a powder (soap) forms which causes intercoat peeling in large sections.  Aluminum does not usually require a special primer.  If white, powdery rust is present, remove with steel wool and dust off before painting.

Does iron require a different primer?
     Often, iron and steel (ferrous metals) require special primers.  Primers for ferrous metal perform two specific functions.  First, they inhibit corrosion (rusting) and secondly, they facilitate adhesion with the more sophisticated industrial products like solvent-based epoxies and urethanes.  Ferrous primers are available in a broad range of application and surface preparation techniques.  Some require sand blasting to "white metal", while others, like "rust convertors", may require only limited physical preparation.  Most manufacturers will give thorough product recommendations when it comes to painting systems for ferrous metals.

Are there special primers for wallcovering?
     Many manufacturers have formulated special primers and sealers for use under wallcovering.  These primers and sealers help with adhesion, uniform appearance of the wallcovering, make it easy for paperhangers to "double-cut" and facilitate the removal of wallcovering when it comes time to strip the paper.  ALLPRO also offers a variety of private label wallcovering primers and sealers in addition to its broad line of wallcovering adhesives.
SURFACE RECOMMENDATION
New Wood Prime with quality acrylic latex or oil primer; use stain-resistant primer for cedar, redwood, mahogony or other "staining woods"; oil-based primers are better for severely staining woods
Repaint Primer usually not needed unless paint is very chalky or bare wood is exposed (see below).
Weathered Wood Thoroughly sand weathered or exposed wood; dust off the surface; then apply a quality latex or oil primer before applying paint.
Masonry Apply masonry sealer first if surface is very porous; otherwise no necessary.  In repaint situation, use sealer only if old paint is removed by scraping or wire brushing.
Ferrous Metals Apply one or two coats of acrylic latex or oil-based corrosion-resistant primer before painting.
Aluminum, Galvanized Steel Primer not necessary unless surface has galvanized rust.  If rusted, follow recommendations for ferrous metal surfaces; remove white, powdery oxide on aluminum surfaces with steel wool and dust off before painting.

In conclusion, please remember that the purposes of priming include:  sealing the substrate from stain bleed-through, providing a uniform surface for topcoating, and to help assure a tight bond between the paint and the surface to be painted.  Proper surface preparation and the correct primer will result in good paintjobs!