Hi everyone! Lately I noticed some brown marks or discolorations on some of my flat-finishes jets, some are rounded and some are more dispersed and irregular in shape, found on different areas of the jet. I have tried to remove them by gently brushing it with brushes dipped in tap water, and also by gently scrubbing them with wet rags but to no avail. What exactly are those? How do I get it of it completely? Thanks in advance!
I tried to attach pictures here but i just couldn't get it done.