Expect controlled variation
Weathering steel changes with light, orientation, rainfall and exposure. Approve a practical range of appearance instead of one screen color.
Check the surface consistently
Use the same viewing distance, lighting note and sample size for every inspection. Record areas of bare mill scale, runs, contamination and uneven development.
Keep appearance separate from grade
A visual check cannot replace a material certificate or dimensional inspection. Keep the patina review, material documents and fabrication checks as separate acceptance items.