I did this on my first CO2 that I bought a few years ago. I took all the bladders out and hung them up to dry. Once the sand and bladders dry out you inflate the bladder and shake the sand down to the valve and blow it out. Washing out also works. Fill the bladder with about a litre of water, inflate and flush out. This is not easy with the leading edge.
It's a bit like getting the last bit of wine out of a goon bag.