Polystyrene is just polystyrene, lightweight balls lightly stuck to each other.
a Quote below from the boardlady
boardlady.com/maintenance.htm"Always leave the bag open, until the board is totally dry!
And for you windsurfers who listen to my preaching about opening the vent plug when not on the water: if you open the plug, then stick the wet board into a bag and zip it shut, the moisture from padz and straps will find its way into the EPS foam core. Don't go there!
And speaking of vent plugs: opening yours whenever you are off the water is the single most effective thing you can do to prolong the life of your board. Leaving it closed will cause the EPS foam core to expand and contract at least once a day. You take any substance, no matter how stout, and you push and pull it continually, at least 365 times a year, and it will fail. Guaranteed!
It's not that much, you say? I just measured my Trusty Old 9'-4": typical summer day, near sea level, early morning, cool 60 degrees, it was 4 15/16" thick. Then the day warmed to 89 degrees, a thermal low developed, and the board expanded to 5 3/16". That is a whopping 1/4" change, or 5%!!Get into a routine, where you tighten the vent screw just before you put on your harness, or some such thing, and it will become second nature, and you will not forget. But DO IT!"