I would like to make the windows of a freeware airliner's VC a little bit dirty because I find that adds to the realism a great deal (like the brilliant YS-11). Can anyone tell me how I can do this please?Thanks

I would bet that has more to do with the modeling than the painting.

It depends on the Aircraft Model.If a transparent Textuere is still assigned for Window Polygons of the interior VC View then this texture with transparency Factor defined by alpha channel contains dirt or maybe "painted" reflections of the Cockpit interior. In this case the Texture can be found and reworked in the textuere-folder of the Aircraft. Otherwise the Aircraft Model must be reworked and recompiled first at the aircraft design-tool (I.E. Gmax).