The Sigma finish
Sigma EX lenses, and a few other Sigma lenses as well, are painted with a matte black finish unlike that of any other camera lenses. The Sigma finish is more opaque than ordinary wrinkle paint, and completely prevents any reflections from the lens barrel. Its looks are very distinctive, because the paint contains very small reflective specks, visible only at close range. Examined with a magnifier, the surface of this finish looks grainy, instead of smooth. This explains its unusual "feel" of porosity, or velvety surface, when touched.
Unfortunately, this finish has two main drawbacks. The first is that it easily becomes dirty. Handling the lens with a slightly dusty or soiled hand, for instance, is guaranteed to leave whitish or greyish dust prints. They can be removed by wiping with a wet towel (of a type that does not shed dust or fibres, or you will end up making things worse). Usually, dirt-prints cannot be removed effectively by wiping with a dry hand.
The second, and more important, drawback is that the black paint ends up peeling off when the lens is handled frequently and/or roughly. For example, my second-hand 50-500 mm came with spots on the tripod shoe that were devoid of paint. By using my fingernails and a wood chopstick, I have been able to peel away the black paint from most of the tripod shoe. The paint peeled off easily in soft flakes.
I have seen worn out spots on several other used Sigma lenses, so this is a frequent occurrence. The zoom and diaphragm markings of Sigma lenses are printed, not engraved, so if the matte finish peels off from these areas you will lose the markings as well. In current Sigma EX lenses, the distance scale is covered by a transparent window, so you don't have a problem here. Fortunately, in most instances I have seen, when the paint peels off a metal surface a more durable, standard-looking black anodized finish emerges under the paint, and the damage is not visible from a distance. This anodized finish seems to be quite durable, and shows no sign of wear. Plastic parts are likewise moulded in a black material. However, I did see the shoe of a 50-500 Apo DG (of the latest variety, with longer tripod shoe and micro-ribbed rubber rings) in which the paint peeled off to reveal the shiny silver-coloured bare metal - so the problem can get much worse than in my own lenses, and it can happen in the newer models as well.
I have often questioned the wisdom of Sigma in using an inferior finish on lenses of superior mechanical and optical quality. I don't have any good explanation of their motives. I would rather use ordinary-looking lenses with a durable, easily cleaned finish, than unusual-looking but visibly worn-out and/or dirty ones.
A careful examination shows that the Sigma finish has been changed at least three times through the years. There is a finish with small reflective specks, one with slightly larger specks, and a third with no visible specks (or extremely small ones). The black background also differs: deep black on some lenses, greyish on others. This suggests that Sigma has been playing with the finish, possibly trying different formulations to make it stick better. I own a few Sigma lenses that I purchased as new within the last year and a half, and none of them show any peeling finish. Thus, this problem is unlikely to happen immediately after the purchase, especially if you take good care of your lenses. Nonetheless, the Sigma finish is definitely more delicate than the traditional black anodizing and other types of lens finish, including the Nikkor wrinkled paint present on larger and/or more expensive lens types. It is possible that the Sigma finish adheres better to certain materials. Modern Sigma lenses have external, painted parts made of plastic, aluminium alloy or magnesium alloy (often, all three in the same lens), and it may be difficult to formulate a paint that sticks equally well to all these materials. It is also possible that the Sigma finish becomes softer and more easily damaged after a few years, perhaps more so when exposed to high humidity or chemicals.