Over the years I have been asked over and over again should exterior natural stone be sealed, and my answer has almost always been, NO. There are several reasons for not sealing exterior stone not to mention all the failures I have seen that can be traced back to sealers.
The following are my reasons for not sealing exterior stone:
1. Internal Moisture- I don't care how dry you think the stone is there is always internal moisture within the stone. This moisture can become trapped in a sealed stone and cause it to flake and spall. Now some of you are saying but I am using a stone impregnator this is vapor permeable(it can breathe). The key word in that sentence is vapor. Impregnators which are vapor permeable will allow vapor to escape, not liquid water. Due to the fact an exterior environment will go though not only temperature fluctuations but also humidity fluctuations causing vapor to condense into a liquid. This liquid will attempt to escape the stones pores and hence will cause pressure and the stone will flake or spall.
2. Freeze Thaw - In locations where the temperature drops below 32 degree Fahrenheit the liquid described above will freeze. The frozen water will expand causing the stone to again spall or flake.
3. Soluble salt migration - Most exterior stone is set on some type of substrate. This substrate can be a slab, directly on the ground, sand, brick or concrete wall etc. These substrates contain soluble salts which will penetrate into the back of the stone carrying these salts in a liquid form. This liquid will than migrate to the surface. If the stone is sealed than it will become trapped and the stone will flake or spall.
4. improper application- I have seen numerous failures where the sealer was not applied correctly. Leaving sealer on the surface, applying the sealer while the stone is still wet, applying the sealer to soiled stone, etc. are all improper applications.
I am using the word sealer above which is referring to stone impregnators or penetrating sealers.
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