Follow the stain upstream

Compare the highest visible origin with the path below it. Does staining begin at a joint edge, a paint chip or an unrelated attachment? Look for recurring discharge at the same location in existing photographs taken after different rain events. A broad orange fan below one tiny origin suggests transport, whereas corrosion distributed across a damaged surface suggests a wider affected area. These are investigation clues, not a severity scale.

Ask what material remains

The engineering question is the condition of the substrate and any load-carrying connection, not the area covered by colour. A qualified inspector may need controlled access, appropriate surface assessment or dimensional examination to establish pitting or section loss. Identify which metal and finish are actually specified. Do not assume an orange mark means a galvanized layer has entirely disappeared, or remove protective material merely to obtain a cleaner photograph.

For example, staining that starts below one attachment and spreads over a cover calls for inspection of that attachment and the runoff route. The cover's large stained area should not become the assumed area of metal loss in an engineering estimate.

Choose the next investigation

Send the source hypothesis, affected component location and whether the joint performs a structural, sealing or protective function. Ask the responsible reviewer to distinguish stain removal from examination of the joint itself. Cleaning alone cannot close an unresolved source, and painting over runoff can hide recurrence. No MARSTON corrosion allowance or permissible loss is established here; an unresolved critical connection requires its own equipment-specific disposition.