Hi
I do a lot of this kind of work for insurance companies... people who have had paint thrown up their house or driveway.
I don't use chemicals unless absolutely necessary and have had great success, but you are correct, a cold washer will not remove the paint. You need to apply steam to get good results.
The only chemicals I use are a citrus based graffiti remover and if it needs a heavy hand, a product called 'ghost' which removes the shadowing left when the paint is removed.
I have attached a couple of photos as an example.
If this is not the way you want to go, then sandblasting or replacement are the only other options I'm afraid, although the former will still not necessarily remove all traces.
Hope this helps
Andy