I have 3 corny kegs of 3 gallons. When finished with beer in them I open the lid in the bathroom and use very warm water and finally hot water from the bathtub faucet, rinse well, take off the dip tube and rinse hot water through it. Then hold upside down and drain out nearly all the water. In the kitchen I add about 1/2 cup of dilute Star San (3ml / 64 fl oz ) into the keg, put the cap back on and roll it around a bit, moving Star San all over inside, bottom and top and then take the lid off and hold it upside down in the sink and drain out what Star San is in there, holding upside down for less than a minute. Re-lid and while the gas release valve is held in UP position add CO2 for about 15 seconds, drop the gas relief valve back down to closed and shut off the gas to the keg. It is now ready to receive some new beer, or store for a while. Remember CO2 is heavier than air and can actually "sit" in a container until disturbed. If filling from the bottom, the level of CO2 moves up the vessel still protecting the product with a blanket of CO2.