That's news to me. I've been painting for 20 years and I've never heard that one. Is there some clue here about what type of plaster that was used?
I was taught to prime unpainted plaster with an "alkyd enamel underbody". Once dry, this primer can then be painted with almost anything.
In leu of that, I would try to use chlorine bleach to see if it will remove the stain. Test it first in an inconspicous area to see if the bleach itself will leave a stain.
I'm curious about this plaster. If you can find out any details, I'd like to know.
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