Why does a red spot appear here?

[Otto J]

Hi, I have installed drywall on the ceiling and then plastered it with gypsum. Near where one of the screws for the drywall is located, a red stain has appeared. I have applied two layers of plaster but it is still just as visible.

There are several screws in the drywall, but only this spot has gotten the red mark. The gypsum is screwed into a wooden stud.

Anyone have a good guess about what's happening?

 

[pacman42]

It could be rust breakthrough, as it doesn't necessarily need to become rust-colored. Is the screw touching anything that could cause galvanic corrosion?

 

[Otto J]

It could be rust breakthrough, it doesn't necessarily have to turn rust-colored. Could the screw be touching something that's causing it to rust galvanically?

As far as I know, it only touches the plasterboard and the wooden beam. Do you know how I can paint over that spot?

 

[pacman42]

As far as I know, it only touches the drywall and the wooden stud. Do you know how I can paint over that stain?

Blocker paint.

 

[Gbgsd123]

Your local well-stocked paint shop has stain-blocking spray in a can, a gold to have at home👍🏻
If you live in Gothenburg, I recommend Ljungdahls färg, which has 3 stores in the Gothenburg area that have it.