I recently picked up a sign, which had obviously spent its life in an urban environment, but although it was plain lettering, it was from the late 19th Century. It had no form of damage, and still had some shine. Its biggest problem was that there was a general discolouration to the surface, and dense black 'sooting' around the edges, which neither a domestic scouring cream, or phosphoric acid would touch. (I have tried T-Cut in the past, but felt that actually put very fine scratches in the surface.)
I remembered I'd got some professional car paint cutting paste in the garage, called 'Farecla G3'. This turned out to be fantastic when used with a damp cloth, removing the discolouration/surface film, and the heavy edge blackening, with just a little elbow grease, absolutely transforming the sign! (I'd perhaps be cautious about using it on fine transferred images, but for something like this, it was ideal!)
