On Mon, 14 Jun 2010 17:34:08 +0100, Another John
wrote:

I have an old tin of blackboard paint, and
whereas ordinary paint would solidify over the course of time (in this
case: about 30 years! :-) ) this paint has separated out, into a
liquid layer of thin black stuff, over what seems to be black concrete.

Trying to answer two posts at once: I haven't got a tin of blackboard
paint, only a tin of matt black paint so I haven't been able to make
two boards and compare them for scuff-resistance. So far I haven't
noticed a problem but it is early days and not in a position where it
gets knocked easily. Nor have I had the tins for thirty years, so I
can't say if there are any differences further down the line.

I certainly don't consider myself an expert so I am happy to be proven
wrong. This all began when I bought a tin of International black matt
paint which said "suitable for blackboards" but then I noticed they
sold a special blackboard paint. I asked them what the difference was
and even they weren't sure! Since then I have seen some shops selling
one tin (of other brands) that is labeled as both matt
black/blackboard so for those manufacturers the matt black and
blackboard paint is one and the same. Clearly with other
manufacturers, e.g. International, they are two different products.

The suggestions here that blackboard paint should contain an abrasive
made sense and perhaps that is the sediment you saw? But who knows how
the formulations have changed in the last thirty years?

As far as the international paints go, neither they nor I could really
tell a difference but YMMV! I painted onto mdf with a roller if that
makes any difference?