Remove the filter for the winter, and throw it away. Replace next season. Be sure to double check the cartridge housing for water later in the blow-out process (if that's what you do). Last time I winterized, the the last thing I did was run the pump briefly to expel water from it--and it dumped about a pint of water into the cartridge housing.
2017 Cedar Creek 38CK Hathaway Edition
2015 Ford F-350 6.7 CC 4x4
Full-timing since 5/2/2016. Roam Sweet Home.
FROG Int'l Rallies: 2012, 2013, 2014, 2015, 2016