I am in Texas and have those stones (flagstone) also. They should not be coated with any type of coating. The only thing that may need to be done is a cleaning. They can be easily cleaned with a light solution of muriatic acid. I use 25% acid to 75% water. Always pour the acid into the water, not water into the acid..... It is not unusual for the stones to reach 140 - 150 degrees F on a hot Texas day. Slight cracks along the mortar joints are common due to the extreme heat that the stones can reach and maintain. These are natural stones and are cut to different thickness....usually 1 1/2 to 2 inches. I have seen pools that people have tried to coat for a shiny appearance. They become slippery and the coatings start to peel off after some intense heat.....again, these are natural stones that are cut down to size..