Coatings: Composition, Chemistry, and Selection
A paint is four things in a can: a binder that forms the film and does the sticking, pigment that provides color and hide, a liquid carrier that makes it applicable and then leaves, and additives that control everything from mildew growth to how long the film stays open. Understanding which of those four is doing the work in a given situation is what turns product selection from brand loyalty into engineering. This chapter covers binder chemistry, sheen, coverage arithmetic worked out step by step, and how to read a technical data sheet. The next chapter covers primers, specialty coatings, sundries, and California's VOC rules.
Binders and What They Are Good For
The binder, also called the resin, is the component that binds the pigment together and bonds the film to the substrate, and it is the single largest determinant of how a coating performs. Waterborne latex paints have largely displaced traditional oil-based alkyds in architectural work because they dry faster, clean up with water, and have far lower odor, which matters enormously on occupied jobs. Within waterborne coatings the distinction that gets tested is 100 percent acrylic versus vinyl-acrylic. A 100 percent acrylic binder gives better exterior durability, adhesion, and flexibility, which is why a coastal home in full sun needing an exterior repaint that keeps its color and stays flexible gets a 100 percent acrylic exterior latex. A vinyl-acrylic, commonly sold as a PVA drywall primer, is an interior product with no exterior durability, and using it on exterior soffits to save a trip to the store is a failure waiting for the first winter. Alkyds still have a place. They level beautifully and cure hard, which is why an owner may want the flow and hardness of an oil enamel on interior trim. Their weakness is yellowing: alkyd binders yellow as they cure without light exposure, which is why white alkyd enamel inside a linen closet turns distinctly yellow while the identical enamel on the hallway trim does not. Where the job is occupied and must meet California VOC limits, a waterborne alkyd or urethane-modified acrylic enamel gives much of the flow and hardness without the odor or the yellowing. A stain differs from a paint in that it penetrates the substrate and lets the wood grain show, rather than forming an opaque film on top of it.
Two-Component Coatings and Metal Systems
Epoxies and polyurethanes cure by chemical reaction between two components rather than by evaporation, and that changes the rules. The mix ratio is not approximate; cure depends on the exact ratio of the two components, so estimating a quart-sized batch by eye produces a film that never fully hardens. Pot life is the working time after mixing, and a five-gallon epoxy kit mixed at eight in the morning and still being brushed at noon on a warm day has passed its pot life and will not cure properly. Amine blush is a greasy, hazy film that forms on an epoxy cured in cool, humid conditions, and it is washed off before the next coat rather than coated over. Epoxies are hard and chemical resistant, which makes them the usual choice for concrete floors, but they chalk under ultraviolet light, so an exterior steel canopy specified in epoxy needs an aliphatic urethane topcoat over it for gloss and color retention. On structural steel, an inorganic zinc-rich primer protects galvanically: the zinc corrodes preferentially, sacrificing itself to protect the steel underneath, rather than simply forming a barrier. For less demanding work where a property manager wants painted steel handrails recoated in a single visit, a direct-to-metal acrylic primes and finishes at once. Interior steel door frames that will be struck by carts do best with a rust-inhibitive primer followed by a hard enamel or waterborne urethane finish. Two specialty metal products round out the family: an intumescent coating expands and chars in a fire to insulate structural steel, and a tie coat is applied between two incompatible systems, such as an existing alkyd and a new waterborne acrylic, because it bonds to both chemistries.
Sheen, Hide, and Color
Sheen is a function of the ratio of pigment to binder. A flat paint carries a high pigment loading relative to binder, so its surface is microscopically rough and scatters light instead of reflecting it directly, which is exactly why flat hides surface imperfections better than a semi-gloss of the same color. The trade-off is durability and washability: semi-gloss and gloss have more binder at the surface, forming a tighter, harder film, which is why they are the usual choice for kitchens, bathrooms, and trim. A hospital corridor that will be washed repeatedly with disinfectant should be specified for high scrub resistance in a tight film. Low-sheen films have a specific defect worth naming: burnishing, the appearance of shiny patches where a cleaner has scrubbed a mark off a flat wall, is the polishing of a low-sheen film by rubbing. Color introduces its own arithmetic. A deep-base paint starts with very little white pigment in the can, because the white would dull the colorant, so a deep red over a white primer can still look uneven after two coats. The standard fix is to have the primer tinted toward the finish color, which reduces the number of finish coats needed. A gallon of deep base that receives a large volume of colorant at the store also behaves differently: expect longer dry times, a softer early film, possible burnishing, and added volatile organic compound content from the colorant itself, which is the honest answer for a client who asks for a zero-VOC interior paint and then chooses a deep color. If a finished wall reads lighter than the sample, the first thing to check is whether the film was applied thin enough that the substrate is showing through.
Coverage and Film Thickness, Worked
Every material estimate is one of two calculations, and both appear on the exam. Gallons for a single coat is area divided by spread rate. A wall of 1,400 square feet with a paint rated at 350 square feet per gallon needs 1,400 divided by 350, which is 4 gallons. For two coats, multiply the area by the number of coats first: 2,100 square feet at 300 square feet per gallon for two coats is 2,100 times 2, which is 4,200, divided by 300, which is 14 gallons. Always round up to whole containers, because you cannot buy a fraction: a 600 square foot ceiling at 400 square feet per gallon needs 1.5 gallons, so you buy 2. On a rough or porous surface such as bare masonry, texture and absorption soak up more paint, so the effective spread rate falls well below the label value, and the first coat on bare porous stucco covers far less area than later coats. That is why estimators price the first coat separately. Film thickness is the second calculation. Wet film thickness is what you measure with a notched comb gauge immediately after application; dry film thickness is what remains after the carrier evaporates, and the two are related by the coating's volume solids. Apply a coating at 4 mils wet with 50 percent solids by volume and the dry film thickness is approximately 4 times 0.50, which is 2 mils. Working the same relationship in the other direction gives theoretical coverage: a coating at 50 percent volume solids yields roughly 800 square feet per gallon at 1 mil dry film thickness. Real coverage always falls short of the theoretical figure, because losses to overspray, surface texture, and material retained in equipment are not included in the theoretical number. Stretching a gallon rated for 400 square feet across 500 square feet produces a thin film with poor hide and early failure, which is the opposite of economy.
Reading the Data Sheet and Storing the Product
A technical or product data sheet tells you spread rate, recommended wet and dry film thickness, minimum and maximum recoat times, application temperature range, thinning limits, and the substrates the product is intended for. It is the document a specification is written against and the document that decides an argument on site. Two figures on it deserve extra attention. Volume solids, discussed above, is the best single predictor of long-term film performance when comparing two exterior acrylics of the same price and sheen. And the maximum recoat time on a two-component product is as binding as the minimum: if the second coat goes on after the maximum, the cured surface must be abraded or the second coat may not bond at all. Storage failures show up as product failures. Latex paint left in an unheated truck through a hard freeze may return coagulated lumps and stringiness that will not stir out, and that material is scrapped rather than strained. A partly used gallon of alkyd enamel that has skinned over is salvageable: cut the skin out in one piece, strain the paint, and reseal the can. Paint stored on the job in an uninsulated trailer through a hot California summer loses shelf life and thickens, because sustained heat accelerates the chemistry inside the can. Finally, incompatible coatings must not be combined: applying a strong solvent-based lacquer over fresh latex lets the solvent lift or wrinkle the film underneath, and a small test area is cheaper than a wall.
Last updated: September 2026