When most people think about sealing stone tiles, they are, for the most part thinking about sealing the top surface and nothing else. But as we shall see there are times when it is actually beneficial to seal all six sides of the stone; that is the front, the back and the 4 vertical sides.
 
The main reason for applying a sealer to a stone tile is to add a degree of stain resistance to the finished (or visible) surface of that stone. However there are occasions where sealing the back and also the sides can help prevent other issues.
 
Consider a polished stone like a marble. The stone itself is porous to some degree, the polished surface of the stone is actually, effectively less porous than the rest of the stone. The reason for this is the polishing process, which creates a much denser, high-tension surface than that of an unpolished marble.
 
Some of these stones are quite susceptible to bleeding: Typically, the short edges are saw-cut and much less dense than the surface, when the tiles are grouted, the relatively high porosity at the edges can absorb the water from the grout. If the grout is a pigmented one, then the colour can bleed into the marble. The result is an effect we call picture-framing; basically a stain all around the edge of the tile. As this 'stain' has come from the edges, and from the full depth of the tile joint, it will be deep within the tile and virtually impossible to remove.
 
Many people are increasingly aware of the role a sealer can play in protecting the surface of a stone from staining, so it is a little strange that so few consider the effect of grout on what is a much more porous and therefore more 'stainable' surface; the edges. This problem can be significantly reduced or even avoided altogether by sealing all sides that come into contact with wet fixing materials, in other words - all six sides.
 
It is perfectly acceptable to use the same sealer for the back and sides as you intend to use for the surface. However the role of the sealer in the exposed, surface-finish will be long-term strain resistance. Whereas all we are looking for on the back and sides is a temporary stain and water barrier, once the adhesive and grout has cured, the risk of bleeding is no longer anywhere near as high; once the grout has cured, and the water has evaporated, then the problem has been prevented. So we do not necessarily need quite the same degree of stain protection and we don't need a sealer with the same longevity as we may want for the surface. In which case, we may be able to use a much more economical sealer on the back and sides, reserving the premium (more expensive) one for sealing stone on the exposed or visible surfaces.
 
Make sure to apply the sealer correctly. Pay particular attention to removing surplus sealer before it dries so as to prevent leaving a residue which may act as a bond breaker.