In uk.d-i-y, Andy Hall wrote:
Maintenance-wise the slate is very good except for a white stain
caused by a dripping TRV. One of these days I'll get round to finding
out how to get rid of the stains, but not before I've replaced the
TRV :-).

Try this in order....

A wipe with a solution of Lithofin Wexa - about 20:1. It's a good
cleaner and you don't need to mix up very much - maybe a litre of
solution for quite a large area.

If that doesn't work then go for a scrub with some brick acid. Not
near the limestone of course.
That should move it. Then rinse thoroughly and go for the Wexa.
Rinse again and lightly polish.

Thanks I'll file that advice away for when the time comes. I've used
Lithofin products and generally they're very good.

--
Mike Barnes