clarke

I fill mine to about 4-5 inches from the top, quite heavy but it gives you loads of water to clean sponges in, ring out the sponge pad on the rollers till most of the waters gone, do a rough wipe over the grout to smooth it over and get it nice and even in the joints, then rinse again and wipe in one direction for as long as poss before it starts to smear.
If you get it right you dont need to buff anything off afterwards.
The sponges dont last that long even if you rinse them well after every time youve finished (they fall to bits round the edges) so its a good idea to have a spare one on board.