Large flat surfaces are quite tricky to do good NMM on. Smoothness of blendings become extra important, since you have such large areas over which to blend, and it can be tricky to make the areas visually interesting when there are no filigree, markings, etc. On the other hand, the large areas gives you plenty of space to \'go crazy\' on. If you want them to be shiny you can make some quite extravagant reflections (follow Arthur\'s advice and look at real metal to see how it\'s supposed to look). If you want them weathered or such, you can make patches of oxidation or use freehand to paint scratches, nicks or other type of battle damage, to make them look more interesting.