Cleaning Granite Cills

Hi,
I have granite cills x 3 at front of my house and they are filthy I suppose from pollution over the years. Was wondering if anyone knows the best technique for cleaning them apart from sandblasting, or is sandblasting the only way.
Thanks,
Randombloke.

I use a power washer on mine every 2 years & they come up fine. Would yours be so bad that they require sand-blasting ? I doubt it - try the power washer first ( rent from local tool -hire or purchase one in Argos for 200 euro)

Sanblasting is really a last resort and is very much frowned upon in building conservation as it effectively removes the exterior surface of the stone, albeit a small amount. As suggested a power washer would be a good alternative. Failing that there are commercial solutions available for cleaning stone, which usually comprise a strong acid or solvent solution, depending on the application. You should be able to source this from a monumental sculptor as they are often used for cleaning gravestones etc.