Countertops that are made of solid stone, like granite or marble are able to be Kashered for Pesach. Other synthetic or composite materials (including granite composite) can not be Kashered and should be covered.
Granite or marble countertops can only be koshered using Eruy Roschin, which means pouring boiling water from a pot that has been heated on a fire. Actual water is needed to kasher, not steam. The concept of using a steam machine is that it will boil water and spray that water through a hose creating a neat and efficient method to accomplish Eruy Roschin. However, if the steamer only produces steam and not actual boiling water, it can not be used to kasher, even if the steam is hotter than 212 f. STAR-K has tested many models of steamers available commercially and have found that almost all of them will only produce hot steam and not boiling water.
Another concept that many people have asked about is to spray steam which will then condensate into water and then continue using the steamer to heat that water to boiling temperatures. This theoretically does work, however, it takes significant time to accomplish this on each part of the surface which greatly limits the practicality of this idea.
One last idea is to spray or pour a small amount of hot water on the counter and then use a hot iron or heated cast iron plate on top of the water which will [re-]boil it on contact. This works very well and also accomplishes an Even Meluban. However, it is dangerous and should only be performed by competent individuals.
The STAR-K hotline is available to answer any questions you may have. Feel free to call us at 410-484-4110 between 9am and 5pm and until 2pm on Friday. You can also email us at firstname.lastname@example.org .
Wishing everyone a Chag Kasher V’Sameach.
 See SHU”A O”C 451:5
 O”C 452:5
 See Igros Moshe Y”D 1:60
 We did find one professional grade model that does work, the Tornado VS4 Professional Steam Cleaner, however, it is extremely expensive and not available in retail stores