For years i have used a spray can of Winsor and Newton matt varnish ,they do a satin as well , i use it on oil paintings too , it seems to produce a high quality matt finish on any paint or decal and does not need a heavy coat . it is not cheap however and needs both the can and the surface to be at least room temperature . it is designed for most art finishes , including soft pastel so works well as a fixitive on light weathering powder . my only comment is that it tones down the effect of ink washes but then most varnish coats do . i use humbrol tin varnish on figures but normally in satin , i have had frosting on matt from several suppliers but it seems a combination of cold damp weather and slow drying