why does my white plaster turn black after shocking ? it will not brush away.
Hi darlyzufall,
The black stains sound like they could be black algae or copper stains. The best bet is to get a Jacks Magic Stain ID Kit this kit will identify the stain type as each type of stain has a different prescription for treatment. Once you determine the stain type from the kit please let us know and we will let you know how to treat the stain.