From someone who sees historic marble architectural details fairly frequently and has been to some of the most incredible marble quarries, here's some things to to consider: Marble was once limestone. Under intense heat and pressure, the crystalline structure became very tight, so there is very limited porosity. Properly polishing marble has virtually eliminated porosity. Polished, real marble will not stain, unless something acidic has attacked/dissolved the surface. Marble that came from dolomitic limestone was most often chosen for exterior details as it is more resistant to acidic contaminants than marble from calcite limestone. If there are stains on real marble, its due to loss of polish or the stains are only *on* the surface as a crust or by static attraction. By the latter part of the 20th century, what's being sold and installed as marble often isn't the same material as the real marble seen in the historic buildings. Here's the ugly truth - many types of stone (that isn't a granite) that can be cut smooth and take a certain polish, can be called marble. That's right, a highly polished petrified mammoth turd can be sold as marble. When purchasing from a supplier, be sure to get authentication from the quarry, that it came from the correct spot in the earth to be true marble. The problem of stains and idea of using sealers came about recently when folks started putting these polished rocks in their kitchens and used them as working surfaces. Um, many foods and beverages that are now contacting these surfaces are VERY acidic, resulting in loss of polish and allowing stains. Working with a couple conservators in the UK, who are trying to protect historic, architectural marble and other unglazed masonry from potentially acidic pollutants, penetrating sealers are sometimes used (in very specific situations). One product name I've heard is Lithofin,