Those are the type of buckets you need, and also try and pick up a cheap square edged bucket trowell, an ordinary gaugeing trowell is next to useless.
Had got a square bucket trowel, so part way there.
so if you throw in a few inches of water to clean it out and empty the big bucket as you swill the dirty water around the bottom at the same time, all the muck will come out, one other tip is to scratch a mark on the inside of the bucket when you have determined what level of water is required to have your mix exactly as you like it, then simply fill with clean water to this mark and you know every mix will bebang on the same without having to keep adding a little more powder or a little more water till its right.
Very useful tips - thanks!
One final question... do you throw this 'slurry' dirty water down the drain or put into a spare bucket? Have avoided in drains so far at home, as wary of it setting underground in the drains.