I defuel into another fuel tank which I carry with me in the flitebox. Inside that tank one line goes to the bottom and the other line is short. Attach your drain line to the storage tank line that goes all the way to the bottom. (otherwise you'll have a mess) Then for the first flight of the day, attach your fuel line to the short tank line& turn the tank upside down & eithe rlet it drain or block off the appropriate line and squeeze. To store it just hook both lines together.
That way you are using all your fuel and getting rid of the old fuel with the first flight. And you don't contaminate your fuel jug with burned residue & humid air.
I had flameouts when near the end of the jug of fuel until I started using this method.