Er - it's the stuff that makes whitewash white Les. Whitewash is a mixture of lime, whiting and size. You used to be able to get it from any corner ironmongers but now you probably have to go to a builders' supply firm or maybe an artists' colourman.
You could probably substitute the much more expensive gesso powder. It gives modelling consistency papié maché a lovely surface, but is no good for Punch heads since it isn't resistant to banging and does reduce the strength of the paper. It also adds weight to the head.
It's good to squawk!