though, smoothness. Real carriage tops have texture and in the larger scales out models ones should have too.
I've seen the results of sticking tissue paper to the roof and they can be very effective. Trouble is when I try all I get are creases which looks a bit rubbish. On the real thing you aren't aware of the material used, just that it isn't as smooth as the bodywork.
For the NG coaches I tried something new. Each roof was primed in the normal way and then received an extra coat from about 2 feet away. Then, setting the airbrush to a wide spray pattern, several coats of paint went on from the same distance. The plan was that the paint would be partly dry by the time it hit the metal.
The result is like sandpaper. The paint seems pretty well stuck even though it feels like it will bush off. However looking at the models under the light, I could see brass. To cure this, a couple of thinned coats of Humbrol 69 were brushed on. While this took the edge a little off the texture, I liked the results even more.
If you were happy to spend a lot of time with the spray gun, I bet this trick would work a treat to represent roofing felt - varying the paints used would even give the right colours.