The only thing for helicopter turbine soot is Hempels Aero Gloss. It has a Mil spec, cleans the soot off even old soot behind cowlings. Will not cause corrosion and its safe on composite materials. This is a cleaner polish, designed to be waterless and has no abrasive components so will only take off the soot and oxidised paint only. If it will not come off with Aero Gloss re paint it because it will be stained through to the primer.