What is a primer?

The word “primer” literally means the prime coat, or the first coat.  This derives from the Latin primus, meaning first.  Something applied to a surface before a topcoat is called a primer, even though it might not be the first thing.  The verb “to prime” means the action of applying a primer.

There are many kinds of primers, and so the word “primer” has no clear meaning when it has too many meanings.  Here are definitions of different kinds of primers:

One kind of primer is an adhesion-promoting primer.  This is sometimes called a tie-coat, because it “ties” a new coating to what was on there before.  Some primers promote adhesion by chemically converting the surface, as do the phosphate primers for steel and galvanized steel, or our MultiWoodPrime™, also known as Clear Penetrating Epoxy Sealer™, developed for use on wood.   Some primers promote adhesion by being able to stick to a surface better than the topcoat.  The two-part urethane elastomer waterproofing coatings actually are chemically poisoned and then won’t cure by contact with the water on a wet surface (or even damp wood), whereas our Damp Concrete Primer and some epoxy coatings are actually designed to stick to damp (as well as dry) concrete and then allow an elastomeric caulk or coating, or an epoxy paint or filler to stick to them.  For concrete or stucco surfaces, Smith & Co. makes the Damp Concrete Primer and the Permanent Stucco Primer. 

Another kind of primer is a porosity-sealing primer.  Porous surfaces such as concrete or wood need some kind of porosity-sealing primer so a coating can stick reliably without blistering, bubbling or having pinholes in the final coat.  Coatings are intended to protect.  Pinholes can allow water vapor or liquid water to pass the coating barrier, and this can lead to rot or mold in spaces that the coating was intended to keep water out of.  Concrete surfaces have a range of porosity, from very large [cinder-block, 1/8” or so] to very small [microscopic].  Wood has a microscopic porosity.  Its fibers are very small, and therefore so are the spaces between those fibers.  The fibers are also to some degree loose on the surface, and need to be glued down into the bulk of the wood to give a strongly-attached surface to which other things may be strongly attached.  The best porosity-sealing primer for wood is Clear Penetrating Epoxy Sealer, which does all of that.  Concrete is physically and chemically different.  The porosity-sealing concrete primers are designed to chemically interact with the chemistry of concrete, whereas any kind of wood-coating is (or should be) designed to interact with the chemistry of wood.  Concrete is often damp, and so concrete primers should be water-based or be able to tolerate being applied on a damp surface.  Wood is often damp, but the wood surface fibers are swollen with that water, and consequently nothing really will stick.  You might as well expect paint to stick to an ice-cube, as expect really good adhesion of anything to wet wood.  Wood must be allowed to dry so the wet wood fibers don’t have a film of water covering their surfaces (and thus so coating-resins can bond to their substance).  As the wood dries the fibers can shrink back, the natural porosity opens-up, and any kind of wood primer can penetrate.  Wood porosity-sealing primers have to penetrate, whereas concrete primers may be of the penetrating type or the surface-coating/chemically-bonding type.   The best concrete priming-systems do both:  First, a porosity-sealing primer that plugs the pores and then a surface-bonded primer for the really big porosity (such as cinder-blocks).

There are different kinds of primers for different kinds of metals.  Many chemical elements are metals;  their mixtures are called alloys.    Iron, steel [an alloy of mostly iron], galvanized [zinc-plated] steel, copper and its alloys [brass, bronze], aluminum and stainless steel are the common metals encountered in construction.  Different kinds of primers perform different functions.  Sometimes a chemical surface conversion must be done.  For example, iron or galvanized steel need a phosphate treatment so that other things can stick, and then some corrosion-protecting coating is applied.  The corrosion-inhibiting layer chemically prevents the iron from rusting under the moisture-diffusion-barrier topcoat that keeps the water-soluble corrosion-inhibiting chemicals from being leached away by rain.  Galvanized surfaces only need the phosphate treatment to promote adhesion, because the zinc coating protects the iron from rusting.  For Steel or Galvanized (zinc-plated) surfaces, our Phoscoat is used first, and then the next layer of the coating-system is applied.  Copper and its alloys need to have the physically weak brownish-colored copper-oxide film removed, to expose chemically clean (a light golden color) copper and then many things will chemically bond.  That type of chemistry is entirely different.  Smith & Co. makes a Copper-Brass-Bronze surface-prep Primer for copper and its alloys.

Moisture-diffusion-barrier primers are used in marine applications on wood, concrete or even on fiberglass boat hulls [GRP, Glass-Reinforced-Polyester].  The High-Build Epoxy Paint is an excellent primer of this type, used over anticorrosive primers on metal or on the outside of GRP boat hulls.  A moisture-diffusion-barrier makes it more difficult for water to get into the hull from the outside, and the natural air-circulation inside the boat hull results in what little moisture does get in being easily able to get out.  This naturally reduces the moisture content within the hull, and thereby, for GRP hulls, stops and prevents gel-coat blisters, also known as osmotic blisters or fiberglass pox.   On a house, an oil-base enamel paint serves as moisture-diffusion-barrier primer [as well as an adhesion-promoting primer for a latex topcoat], or the oil-base enamel may be the topcoat itself.  Clear Penetrating Epoxy Sealer (CPES) is a very effective vapor-selective primer on wood surfaces as it prevents the passage of liquid water but allows the passage of water vapor.  This can actually prevent rot in wood, by preventing the accumulation of excess water inside the wood, behind paints, caulks or fillers.  The combination of CPES, an oil-base primer and a latex topcoat is commonly reported by professional painters to “Still look like the day I did it” routinely twenty years later.

Sanding primers are applied to wood, to glue the often-loose surface fibers of wood together, so that a smooth sanded surface can be obtained and a smooth, uniform topcoat applied without a grain-pattern showing.  This will give a final finished surface on cabinet-work that looks the best.  MultiWoodPrime™, also known as Clear Penetrating Epoxy Sealer™, is used for this purpose.   Wood with a “milled” finish may have a “mill-glaze” which is the natural varnish-like resins of wood smeared out and partly cured on the surface.  This is caused by friction-heating of the wood in being cut smooth and to size at the millworks by the planing-blades becoming dull, as they always eventually do.  Sanding such a wood surface may be necessary [typically with 120-to 180 grit] just to allow a primer or coating to stick.  On old, weathered and rough window-sills, a thin film of the Fill-It Epoxy Filler is used on wood treated first with the Clear Penetrating Epoxy Sealer.   The thin film of Fill-It, sanded the next day,  efficiently serves as a “sanding primer” for such very rough surfaces.  One last application of Clear Penetrating Epoxy Sealer the day before painting, that’s now the adhesion-promoting primer that glues the paint down to the surface.

Combination-primers also exist.  Tannin-block primers, either latex [waterborne] or solvent-borne [oil-base or epoxy] are designed to be applied to exterior wood and block the tannin that is normally present in many woods, from bleeding through and staining an exterior latex topcoat.  Tannin is a brown chemical component of many woods, and can migrate to the surface of many paints, causing unsightly staining.  Latex paints applied directly to wood often have this shortcoming.  It is possible to formulate a coating that contains structures that chemically bond to the acid and phenolic structures of the tannin molecules, binding them and preventing their migration. Clear Penetrating Epoxy Sealer contains those structures, thus it is also a combination-primer, for it blocks tannin bleed, promotes adhesion of topcoats to wood, and also glues the surface fibers down to the bulk of the wood beneath, so that there is now a strongly-attached surface to which other coatings can stick.   

----Steve Smith

© copyright Steve Smith, 1972 - 2015, All Rights Reserved.