I tried to seal the multiport valve threads with plumbers goop, but after a day of setting, it blew right out. Three days ago I tried some marine grade epoxy and it is still holding with no leakage at all. I'm hoping that it will continue to hold. Of course, I'll probably never get the multiport valve off again, but at least the tank is full of clean sand and hopefully it will last a few more years before I have to buy a new filter.