On my cellar walls and joists, there is a white coating which can be
described as a cross between paint and plaster. It is quite thick but
most is coming off in large brittle flakes. Am I right in assuming
that this is whitewash? I know that a lime based coating is best in a
cellar as it breathes. Does anyone know how I would make up more of
this?

Water based distemper is 'dusty' so best used only on ceilings where
it wont get rubbed against. I suspect you cellar has been limewashed
many times.

Remove any loose flakes, brush down and then wet the wall until the
water stops being sucked greedily into it.

Limewash is lime putty mixed with water to a single cream consistency.
You will need to stir the limewash regularly while painting cos it
tends to separate out again. Lime putty can be bought by the tub for
about £15 from various places including Jewsons. One tub will make
enough limewash to cover 1000 cellars. Don't buy readymade limewash
from a paint supplier as you will be paying a lot of money for water.

Work the limewash into the surface with your brush because you are
trying to get close enough contact to make a chemical bond. For the
chemical reaction to happen both air and water must be present, so
turn off any heating in advance and give it a spray with water a few
hours after you finish. Air flow might be a problem in a cellar, so
allow plenty of time for the first coat to set before doing the next.
Normally the next day would be OK but in a cellar you might want to
wait until next weekend.

The limewash will be very transparent when its first put on, but as it
sets it becomes brilliant white. Three coats is reasonable, but see
how you go.

Anna
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~~ Anna Kettle, Suffolk, England
|""""| ~ Pargeting, decorative and traditional
/ ^^ \// lime plasterwork
|______| www.kettlenet.co.uk 07976 649862