This is a small walkaround I did of a Czech Hind last year. The Czechs keep their Hinds quite clean as this aircraft shows. However one area that dirties up pretty quick is the area where the exhaust flows. The exhaust stream goes down the fuselage at an angle to a point just aft of the main landing gear bays. The wing roots get a good dose of exhaust staining in the process, the sixth picture shows a bit of that.
Hope these pics help you:
http://www.primeportal.net/hangar/kevan_vogler/mi-35_hind/