It's crazily hydrophilic (loves water) so it gets messy to work with straight away. The moisture on anything is enough for it to leave a sticky film. A walk in freezer would be a great place to do it, get temps right down it'll be dry as a bone in there.
It does stay powdery once you open and close the pack quickly. To give you an idea, the internal bag, is covered in a film of malt, straight away when it comes from Muntons, so double bag it like they do.