Tiger, I can only state my experience, which, whilst not considerable, has at least been gained from 'experience' - albeit short!
I have a 1:96 Belfast that had lived in a loft (before I had her), for 8 or so years. This loft must have had a leaky extractor fan not far away, and there was some residue on the decks Fore of the Bridge section, and parts of the Bridge superstructure. There was only one way possible to remove same, and this was with some kind of 'thinner': however, not wanting to damage her matt paintwork, I choose Surgical Spirit as the more gentle medium, and proceeded to wash all affected areas with this, using an in turn procedure of washing off the Spirit with a warm Fairy Liquid 'wash'. This certainly worked to get rid of the 'deposits', and it did not damage the paintwork. Areas that were not affected by the deposit were washed down with the F.Liquid 'solution'.
This treatment has enhanced her look I feel, but it certainly helped with her having a matt finish (someone else has already mentioned this), and, together with her paintwork having had much time to 'settle', all in all she has a weathering that looks excellent... my only attention will be to seal everything with a satin 'varnish' - to enable a seal to the finish and to make keeping her clean in the future than much easier.
Hope this might be of use.
Regards, Bernard