Just drained a keg of cider. Do I clean then sanitise or is it ok to just sanitise it and then keep it under co2 until I transfer the new batch into it? Possibly gonna put a Pilsner in there.
It's definitely a good idea to clean all booze containers immediately after use, before any crud can stick and dry out. You keg likely has some dried stuff on the sides. So, give it a long soak and then a scrub inside (if you can) then sanitise. Is this a metal keg or a plastic King Keg?
For the sake of the time it takes to clean your kit first, clean every time. You'd be gutted if you didn't and it ruined your all your hard work which you would only find out when you pour your first pint.