What is a primer? (tutorial) (heating, paint, conversion, converting)

[steve327]

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 would be called a primer. 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. You can search the Internet for the particular kind of primer you need for any particular application, by using the word “primer” and a few other descriptive words.

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 the phosphate primers for steel and galvanized steel, or the penetrating epoxy primers such as MultiWoodPrime or Clear Penetrating Epoxy Sealer for 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, after contact with the water on a wet surface, whereas the damp concrete primers and some epoxy coatings are actually designed to stick to dry or damp concrete and then allow an elastomeric coating or an epoxy paint to stick to them.

Another kind of primer is a porosity-sealing primer. Porous surfaces such as concrete or wood usually 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 protect. Concrete surfaces have a range of porosity, from very large [cinder-block, 1/8” or so] to very small [microscopic]. Wood typically has a microscopic porosity. Its fibers are very small, and therefore so are the spaces between those fibers. A common porosity-sealing primer for wood is Clear Penetrating Epoxy Sealer, but wood and concrete are chemically different. The porosity-sealing concrete primers are designed to chemically interact with the chemistry of concrete, whereas any kind of wood-coating is designed to interact with the chemistry of wood. Concrete is often damp, and so damp concrete primers should be used. Wood is often damp, but wood must be allowed to dry so any kind of wood primer can penetrate. Wood primers have to penetrate, whereas concrete primers may be of the penetrating type or the surface-coating type.

There are different kinds of primers for different kinds of metals. Metals are chemical elements; 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, so the iron does not rust. Galvanized surfaces only need the phosphate treatment to promote adhesion, because the zinc coating protects the iron from rusting.

Moisture-diffusion-barrier primers are often used on wood, concrete or even on fiberglass boat hulls [GRP, Glass-Reinforced-Polyester]. The moisture-diffusion-barrier reduces the moisture content in the hull, and thereby stops and prevents gel-coat blisters, also known as osmotic blisters or fiberglass pox. An oil-base enamel paint may serve as moisture-diffusion-barrier primer [as well as an adhesion-promoting primer] under a latex topcoat on a house, or the oil-base enamel may be the topcoat itself. Penetrating epoxy primers may be used on wood surfaces wherein they prevent the passage of liquid water but allow the passage of water vapor. This can actually prevent rot in wood, by preventing the accumulation of excess water inside the wood.

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. Wood from the mill 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 by the planing-blades when those blades become dull. Sanding a wood surface may be necessary [typically with 80-grit] just to allow any primer or coating to stick.

Combination-primers also exist. Tanin-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. Clear Penetrating Epoxy Sealer is another combination-primer, for it blocks tannin bleed and promotes adhesion of topcoats to wood. It 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. Multiprime is a combined-function metal primer.

By using the Internet to search for the kind of primer you want, using words that describe what you want the primer to do as well as the surface to which it will be applied, you will be able to find the right kind of primer for your job.