Re: Can anyone identify the type of stain/stain causing agen
It will require a very close look and feel to determine whether the "roughness" is a deposit of calcium carbonate, or a loss of the surface material (plaster cream) resulting in an etched condition.
If the roughness is raised above the surface, then it could be a calcium deposit that is taking on iron for the yellow color.
Is the roughness more on the vertical walls, or on the horizontal flat surfaces?
It also could an iron staining issue which is adhering to the more porous areas of your plaster surface.
The porous areas are either due to aggressive water or from poor finishing practices, such as adding water while troweling. It becomes a long-term issue.
Not much can be done to rectify if due to a porous or etched surface.