On Thu, 02 May 2019 22:53:02 +0100
Dave W wrote:

You'll probably find that the seating that the washer screws down on
has grooves caused by years of grit in the water, so you might have to
grind them down, in such a way to get rid of the grooves without
widening the radial area of contact if possible.

If you find you're replacing tap washers more frequently then it's well
worth investing in a re-seating tool - a nice fresh face on the valve
seat makes taps work better and last longer between servicing.