Martin, I looked up this kit and I think maybe you are trying to make it like the picture on the box top. One thing, I think you should have put the decals on already, remember that the winter white wash would have been put on over the standard green paint and markings. Do you think maybe some of the patches you've washed off (not sure which technique you used) are quite big and not necessarily in the right places? Try looking up some photos of winter camo tanks with a rough finish and see how it gets worn off, often in smaller and more chaotic patches.

Although it is a 1/35 model, you might find this build of an SU-76 by Steve Zaloga interesting, he did the white camouflage by masking with toothpaste.
http://www.militarymodelling.com/news/article/a-sturdy-su/4632