Being a porous stone you need to be careful what you use as sandstone cleaner. Whether the sandstone has been used as a construction material in your home or if you have outdoor ornaments or even garden furniture made of sandstone, you really should clean it as little as possible; don’t forget that any staining or discoloring due to the natural weathering of a stone is often part of its beauty.
Cleaning Sandstone by Scrubbing
The gentle scrubbing of sandstone with clean water can be an effective way of cleaning it. However, as sandstones weather over time - the minerals in them form a natural protective crust, which scrubbing will simply remove. If you unnecessarily remove this protective crust you will simply encourage further weathering, which will mean the surface will become further degraded.
The result of this is that in the case of stone used as building material - the stones become thinner, losing structural integrity; whilst an ornament will lose its surface definition and relief - making it difficult to see what the ornamentation originally was.
Obviously, apart from removing the protective weathered surface, as some sandstones are quite friable - scrubbing sandstones too harshly will simply cause it to crumble. So if you do have to scrub a sandstone, test the brush you’re going to use on a small and ‘out of the way’ area first, if the bristles are too stiff - get a softer brush. Never use a wire-brush to clean sandstone! Using the softest brush possible will make the job take longer, but it will also be the best preserver of the sandstone.
Traditional Ways of Cleaning Sandstone
Mildew stains can be removed by using a ‘blotting paper’ technique. Using shredded sheets of clean white paper, soak them in hot distilled water and mix them into a pulp; then spread the pulp about 25mm (1 inch) thick over the affected area. Being porous the sandstone initially draws the moisture out of the paper, but as the paper dries it then sucks the moisture back and with it the stain.
If you’ve got an oil stain on sandstone the same method can be used, but before applying the pulp douse the affected area in white spirit then apply the pulp when the spirit has dried. Less favored these days is the removal of oil stains by spreading a paste made of kaolin powder and petroleum spirit or Gasoline. Understandably, using a volatile liquid like Gas is no longer considered a good idea.
Modern Methods of Cleaning Sandstone
Whilst water jets and vortexes are popular sandstone cleaning methods, they are also an abrasive action which can, if not controlled properly, have the same effect as over zealously scrubbing it. Chemical methods of cleaning sandstone can create problems like bleaching the surface, salts being leached out of the sandstone and crystallizing on the surface and worse actually dissolving the sandstone leaving a pitted and eroded surface - sometimes they have to be applied.
Rather than pouring, painting or spraying the chemicals on, these days the chemicals are increasingly applied as a gel or paste to localized areas that need cleaning. That way less of the chemical can spread to areas that you didn’t want to treat and it is easier to identify the area that needs the chemical rinsing off afterwards to prevent it working too deeply.
The very latest chemical solutions for cleaning sandstone can utilize latex gels and bacteria. There are also some specialist companies that can apply laser technology to getting sandstone clean.