Surface Preparation and Substrate Condition
Bridge steel arrives with mill scale, rust, previous coatings, soluble salts, and fabrication defects. The candidate must separate cleanliness from profile. SSPC-SP 5/NACE No. 1 (white metal) and SSPC-SP 10/NACE No. 2 (near-white) define visual cleanliness; SSPC-SP 2, SP 3, SP 6, SP 7, SP 11, and SP 12 describe hand, power-tool, brush-off, and waterjet methods. A frequent trap is assuming dry abrasive blasting removes chlorides, sulfates, and nitrates. It does not do so reliably. Salts must be measured and removed by washing, then rechecked, because trapped salt draws moisture through the film and causes blistering or disbondment. Another trap is confusing cleanliness with anchor profile. The coating data sheet may require 1.5–3.5 mil (38–89 µm) profile; verify with replica tape or a profile gauge, and remember that excessive profile can thin the primer over peaks. Fabrication conditions matter too: sharp edges, weld spatter, skip welds, laminations, and pitting are substrate defects. Grind edges or rely on a stripe coat, remove spatter, and treat pitted rust grade D as a salt-retention risk. Before any of this, confirm the steel is dry and at least 5°F above dew point; condensation during preparation undermines both cleanliness and profile. A workable rule is: clean and dry first, verify salts, blast to the specified visual standard, then verify profile before coating.