The cold water shower faucet (old style with separate faucets to mixer)
continues to drip after replacing both the seat and washer. This is a
trivial job I've done many times over the years and I'm baffled as to how
water can still be getting though.

I've tried a few different kinds of washers (flat and beveled), checked that
the seat is tightened firmly and not cross threaded, swapped it with
another new one though it looked perfectly smooth, and coated the seat
threads with sealant. The valve stem screws down firmly, i.e. is not
stripped and the washer area looks perfectly fine.

I may just have to replace the valve but have to go through a wall to do it,
so any ideas would be greatly appreciated.
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