We had really bad mold on our shower ceiling, I got it steam cleaned off and it was clean, though the guy said that the mold had gone into the paint (there was a light grey stain left on the ceiling after the steam clean). So 7 days after he had steam cleaned I prepared the ceiling for painting, sanded it then used a water based paint especially for bathrooms (has mold preventer thingy in it) and painted it. Now another 7 days later I’ve got these red splotches coming through my newly painted ceiling – whats happening?
Most likely the red spots are stains coming through your recent paint from the subsurface. You will need to seal these areas with a stain blocking primer. An alcohol based shellack primer will work the best if available in your area. Nice thing about the shellac primer is the alcohol, can kill some of the remaining mold in the old paint. If not available use an oil based stain blocking primer.