I would always use a file, ...the problem with using an angle grinder or
grindstone is that you stand a good chance of heating the blade and spoiling
the temper if the metal.
This is a job you only need to do once, you are thinning the leading edge of
the blade, not just putting a cutting edge on it.


--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk