Very tricky inspection situation here for the untrained eye, it looks like a rusting tank surface.
Upon further examination, it shows metal grindings that came off the grinder and then flash rusted, this is not a paint scenario this is sloppy work from the plumbing trade that allowed metal grindings to sit atop a tank, and when unpainted steel is left to dew or rain they rust.
The solution, lite sanding to sp2 or sp3 standards, sp1 solvent wash and then reapply finish, costly mistake for a plumber in the tune of thousands of dollars for only a few square feet.
Preventive measure, fireproof tarps under any grinding that takes place over a painted surface.
LDG Consulting, failure analysis, project specifications, inspection, and control.