Hello,
I need help with an unknown problem on my wall surface. It appears to have white patches/stains that resembles powder.
I can't wipe it off using my finger, and it could only be removed through a damp rag.
Please refer to the attached picture. From this, you could see that the water droplet actually cleans part of it, but the perimeter of the water droplet has the "white powdery substance".
Any advise on this? Thanks a million