Based upon my limited work with NMF I would reccommend the following:
1. Alcad metallic finishes - They are expensive but seem to give the best finish. (you can also mask over them). If you don't want to go to that expense, I would say to go with Model Master metallic finishes. (biggest problem is you can't really mask over them and they need to be sealed when finished) Either can be found at any good LHS or on-line.
2. Alcad requires a gloss black primer to get the best finish (I've used Krylon Gloss Black from the hardward store and it worked well). Model Master can be painted over primer, paint or plain old plastic. The primer or paint should be smoothed with very fine sandpaper because it's going to show any blemishes in the surface it's applied to (Alcad will also show any blemishes - silver coverings tend to do that)
3. I haven't buffed any Alcad as I felt it wasn't needed. Model Master comes in buffable and non-buffable colors.
4. Airbrushing is directed for both products (you spray them directly out of the bottle)
As far as being hard, it probably isn't any harder than airbrushing any other covering, as long as you follow the directions.
Hope this has helped a little