Concrete countertops are prone to staining if care is not taken. Acids like vinegar, wine, citrus, etc. will etch and stain if they are not wiped up within a reasonable amount of time. The amount of time depends on how well the countertops have been sealed. There are several methods of sealing concrete countertops. Since most countertops are found in kitchens, the sealers need to be "Food Safe". If you are not using the counter in a kitchen, i.e. a vanity top, this is less of a concern. There are many sealers available on the market for this purpose. If you do an on-line search for concrete sealers and include "FDA" in the search, you will come up with a good list of possible products. If you aren't concerned with "food safe" sealers, the list will be much bigger. Now for the bad news. No product currently available will protect concrete surfaces 100% that I am aware of unless you go with an epoxy type sealer. I personally avoid these. They are generally difficult to apply and will scratch over time leaving you with an unpleasant removal operation. One of the most effective sealing methods I have found is the use of straight beeswax. The downside to this is the amount of work involved. You melt the beeswax onto the surface (an old iron with aluminum foil over it works well but you need to be very careful). After applying a uniform coat, the wax needs to be scraped down to a minimal layer (a cabinet scraper works well for this). You can then buff the surface out with a wool bonnet. Lots, and I mean lots of work is involved but the finished surface has a great sheen and is very durable. It isn't prone to "smuding" like many other waxes due to the hardness. My arms ache just thinking about it though. The method we currently use in our shop is a FDA approved waterbased sealer, (we have had very good results with several brands) followed by a food safe wax that is buffed with a wool bonnet. We apply 3 coats of the sealer (enough so the sealer is no longer absorbed by the concrete). This is followed by two coats of wax. Another finish material I have recently heard good things about is 100% tung oil, though I haven't got any experience with this method. Hope this helps!
Hugh Griffin - Griffin-Smith Castcrete.