Is the soot on the surface, or is it staining that's penetrated the wall?
For the surface soot, I use heavy duty traffic film remover at about 5 to 10% dilution, and apply it with a hand spray. Watch it though, soot is very very black and a little goes a long way.
The traffic film remover is used with pressure washers, and has sodium hydroxide in it, so it would be a good idea to wear pvc gloves.
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