How can I seal an old multiport valve to its tank??

I have an old, at least 1980s PACFAB Tagelus sand filter. All of the labels are gone except for the one that I can read the the name (above). I have owned the house and pool for three summers. I decided to check the sand in the filter because I have been unable to clear my water. After much struggle I removed the valve by unscrewing out of the fiberglass tank. I took a sample of the sand to a pool store and he suggested I change the sand. I put 350 lbs of sand in the tank and screwed the valve back into the tank with teflon sealing tape around the threads. Water was leaking out around the tank and the valve in a small amount. I decided to attempt to reseal today, but before I could reseal the whole valve blew out of the tank, I believe the threads have stripped. Does anyone know of a way to get the valve to stay and not leak. I was thinking about some sort of glue to glue the valve down into the fiberglass tank. I realize it would be the last time I would be able to take the valve off, but It could possibly last for a few years. Otherwise, looks like I'm going to have to buy a new filter. Thanks for any help.

It sounds like you have the same era multiport valve and problems with keeping the water clear that I do. I posted yesterday asking how to remove it. Now I'm thinking twice about doing that. Do I understand you to say it unscrewed off? I thought mine was a screw-type but can't get it to budge even a little. If you read my post from yesterday you'll see what I've attempted to do. My valve was removed 5-7 yrs ago by the former owners so I wonder if they "glued" it in place. Can't financially swing a whole new unit right now so I may just forgo messing with it. I'm going to ask at the pool store about it. Will reply to you again if I get any words of wisdom. They are very good about trying to help customers do things themselves. They were the ones that said changing the sand was "easy". Riiight...
Did you try any other type of sealant? I remember using something several years ago to put on my pump seal to keep it in place. Pool store or Home Depot may be able to help. Good luck & keep me posted on what happens, we do-it -yourselfers have to stick together!
Jill

Jill,
I may have given you wrong advice about how the thing is mounted to the tank. This thread jogged something and I sort of remember comments about some valves being screwed in. Hayward uses a collar that grabs the valve and tank lips and pulls them together.
As for the valve blowing out that is quite possible. Consider the tank pressure and the number of square inches on the valve and you can one heck of a force trying to push it out of the tank. Because of the pressure the tank expands a little with the pump on and because of this I can't see how any bond or glue will hold. Hope someone has some ideas for you all.
Al

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.