On Ricks Restorations on sky, they had to blast a fairgorund ride that was huge, so had to be done outdoors.

They got a company in to blast it using dry ice as a media. Little frozen lumps of co2 were used, it was blasted and the only residue left was the material that was removed from the metalwork, as the blast media just evaporated.

Cant imagine such a company is as easy to locate and pay in the UK though !