Leaking Sand Drain on Pentair Triton II

My filter is ten years old and works great. The sand drain at the bottom leaks and doesn't thread properly anymore. I've tried a simple PVC plug (3/4"), but the hole is a hair to big. The threads on the sand drain are good. I'm hoping the threads on the fiberglass tank are not shot, but they certainly don't feel smooth.
One possibility is that it is almost impossible to get the threads clear of sand. I've tried teflon tape, but the sand chews the teflon to pieces. I'm about ready to try my anti-seize lubricant, a thick, oily, silver colored paste to try to keep the threads as clean as possible when reinserting the sand drain.

Any other suggestions??

The threads are worn away...The sand drain plug can be slid into the opening without turning.:eek:

Can't help, but hoping to bump your question! Good luck!