Coating Materials, Selection, and Specification
Coating selection starts with the bridge's exposure zones: immersion or splash, deicing-salt splash, atmospheric, and sheltered areas. The specification should name the generic system, primer, intermediate, topcoat, dry film thickness (DFT), surface preparation, and recoat requirements. Common binders are epoxy (good barrier and chemical resistance but chalk in UV), polyurethane (UV-stable topcoat), polysiloxane (color and gloss retention), and zinc-rich primers (sacrificial protection; inorganic or organic). A frequent trap is selecting a product by brand or color instead of by binder, volume solids, and service environment. Another is expecting an epoxy topcoat to retain gloss; it will chalk. Zinc-rich primers generally require near-white blast cleaning and may not be compatible with certain topcoats or slip-critical joints. Recoat windows and pot life are part of the specification, not suggestions. The product data sheet and project specification can conflict; the specification normally governs, but a written submittal or RFI should resolve it before application. VOC limits may restrict thinning and product choice; never thin beyond the data sheet. For quantity and wet-film checks, use volume solids. Worked example: a 5.0 mil DFT coating at 65% volume solids needs WFT = 5.0 ÷ 0.65 = 7.7 mil. Add a spray-loss allowance only if the specification or estimator requires it. Rule of thumb: select the system for the most severe zone it touches, and verify compatibility of every layer.