I have a tap that is dripping and I have looked at it and seen that the
seat needs to be dressed. I have a tap re-seating tool, and when I used
it in the past I ended up calling a plumber. He dressed the seat but
after a couple of years it needs to be done again. The re-seating tool
that I have is cheap and nasty (like this
http://www.toolstation.com/search.html?searchstr=69230 - the cutters
have saw-tooth cutting edges like a router), and I am tempted to get a
better one, but when I remember the problems I had before I am not sure
that a better one will overcome them. My problems we-

1) The tool had a tapered thread that wouldn't engage very positively in
the tap thread, so I could never be very sure it was installed
perpendicular to the tap seat. Some of the tools available have a
parallel thread, but I don't know how to know what thread size I need to
fit my tap. How do I measure it?

2) The worse problem was that when I turned the tool it did not turn
smoothly, but chattered, so that I ended up with a seat with a wavy
surface. I think chatter is a known metal-working problem associated
with cutting brass, so nothing new there. I've found a couple of guides
on the Internet about using a re-seating tool, and they say 'turn the
tool smoothly', and that is just the thing I couldn't do!

Any comments or advice would be very welcome.