Coating Materials and Selection
A coating is a formulated system of binder, pigments, fillers, solvents, and additives. The binder determines chemistry and performance; pigments provide color, corrosion inhibition, and barrier; solvents control viscosity and evaporation; additives control flow, sag, settling, and cure. Thermoset coatings cure chemically-epoxies, polyurethanes, polysiloxanes-while thermoplastics such as vinyl or chlorinated rubber dry by solvent loss. Epoxy types include amine, polyamide, amidoamine, phenolic, and novolac; aliphatic polyurethanes resist UV chalking better than aromatic types. Zinc-rich coatings protect by galvanic action and require sufficient zinc dust; inorganic zinc needs a properly blasted, moisture-controlled surface and can be damaged by too-early topcoating.
Selection depends on service environment, substrate, temperature, chemical exposure, expected life, VOC limits, and recoat compatibility. It is a mistake to treat "surface-tolerant" as "surface-prep-free." Volume solids, not weight solids, determine the wet-film-to-dry-film relationship and VOC. Mixing ratio is critical for plural-component and kits; induction time is the standing period after mixing; pot life is usable life after mixing. Exceeding the recoat window usually requires abrading the surface, and adding thinner beyond the data sheet can cause cure, sag, or VOC problems.
Worked example: for 6 mils DFT at 65% volume solids, WFT = 6 ÷ 0.65 = 9.2 mils. If the product is thinned 10%, assume volume solids fall roughly to 0.59 and required WFT rises to about 10.2 mils; verify with the data sheet and a wet-film gauge.