I'm about to start plastering the inside of a little garden folly. It
doesn't have a damp course, but I don't think it is subject to rising
damp. The first 4' is built of reinforced concrete which at one time
was sealed with a thin bitumastic compound (whatever they used in the
seventies) which is still covering about half the area. There's a ring
beam of very dense concrete above that. And above that, the
construction is concrete blocks with a concrete render coat, very
rough. The outside of the building is concrete rendered.

In the long term, I want to put in some rather fancy plasterwork with
moulded designs. I think the first thing would be to create a
reasonably uniform plaster surface, then add the fancy stuff. The
plaster should be white or white-ish---so it has the look of old lime
plaster.

Is there a kind of plaster that will do this job? How do I prepare
the various surfaces? I hope that a coat of PVA or SBR (or something
else?) would be all that's needed to help new plaster stick on. Or is
it better to try to get rid of the black stuff?