Although sand blasting may be an option, care should be taken prior to any work be carried out. Chances are that the stonework was originally painted due to the stone becoming discoloured from the weathering effect and as such, the weathering effect could have also changed the stones density.
As mentioned the option of sand blasting would remove the paint and clean the stonework in one process however it can also cause pitting to the face of the stone due to it's harsh blasting process. Also if the stone is soft in places, the blasting process can remove sections of the stone as easy as putting a knife through butter.
Power washing can also be harmful if it is used incorrectly, causing whip marks.
My advice is to seek out a restoration specialist. Like ourselves, there are many out there who will be more than happy to offer advice on the best methods including solution based products so that your property will once again become a natural thing of beauty for many years to come.