How to remove grout from sandstone tiles.

I have just tiled an external concrete slab with sandstone tiles. I've applied grout and cleaned up with a sponge, but I didn't get the tiles completely clean. Anyway there is excess grout on the surfaces of the tiles which is hard to remove.
Anyone have any advice on how to get it off? Would high pressure water blasting clean it off and is water blasting a DIY job or would I need a contractor to do it?
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