QUESTIONWhat can I use to harden and seal my sandstone exterior patio?
ANSWERANSWER - I assume you have a class 1 sandstone which is the most absorbing and has the lower physical properties within the quartz geological classification.
This stone is suitable for many applications, including back yard patios where there are not freeze thaw conditions. Although this classification of sandstone will be more susceptible to various types of damages or has a greater propensity to be damaged of various types if it is not installed correctly and maintained correctly. Those common damages are its susceptibility to staining due to its high absorption and its susceptibility to spalling (surface deterioration).
There really isn't anything sold specifically to make the stone more hard or dense. There are a variety of sealers that are temporary solutions to minimizing damages, but they won't compensate for improper installations or conditions where the stone is being subjected to excessive moisture.
In the lapidary-gem world they do use “water-glass”, which is also known as sodium silicate, by soaking the stones in the liquid to stabilize crumbly stones. I have never heard of it used with building stone.
I have a class 1 sandstone in my pool yard and on my barbecue counter and I just seal it with a enhancer sealer such as Miracle Sealants Seal and Enhance Stone Sealer and Enhancer about once a year. It enhances the color and seals to make it more resistant to staining. http://miraclesealants.com/index.php?option=com_virtuemart&view=productdetails&virtuemart_product_id=34&Itemid=213. I haven't had any staining problems. I have had spalling problems where it was installed on the spa coping that didn't have a waterproof membrane installed under it as there should have been.
The most damaging problem with this type of stone is that if stone is subjected to excessive moisture particularly from beneath the stone, the moisture migrates through the concrete, stone and setting materials picking up minerals, which are forms of salts that dissolve in water. The moisture is driven to the stone surface where the moisture evaporates that causes the minerals to predicate. As they crystallize and expand near the stone surface they cause the stone surface to spall/deteriorate leaving the efflorescence residuals. We have investigated many major failures where improper installation not allowing for water to adequately evacuate from beneath the stone surface or at the surface has caused major spalling and efflorescence problems.
There is a product that I ran into that I never used or tested that sounds almost too good to be true manufactured by Sinak Corporation called Sinak Sealer HLQ-125 www.sinak.com. It is suppose to penetrate and react with the mineral compounds and/or siliceous materials to form an insoluble silicate structure that waterproofs and hardens the stone. Again it won't compensate for an improper installation or for stone that is being subjected to excessive moisture.