Sealing LimeStone (http://g603.net)

I have two exterior decorative walls about 8 feet high. They are constructed of what we call Austin Stone in my part of the country. It's an off-white limeStone (http://g603.net) , I think. A trip up the ladder showed that the walls are cracking on the top and (http://maplered.com) are showing signs of spauling. :( We are in the middle of a major dry spell, so this may be a good time to seal the tops of the walls. What do you recommend to seal them?