Even though a gravestone may be made up of materials like those found in the exteriors or interiors of building (i.e. marble, granite, or other stone), the same techniques that would be used to clean and protect the building cannot be used in the cemetery. Unlike buildings, gravestones are also exposed on all sides to the environment and must be able to allow air and water pass through it. The introduction of a sealant to the stone will prevent this process and will cause later damaging effects to the stone. Keep in mind that in many cases the stone is in direct contact with the soil and anything in the ground can be wicked up into the stone. Sealing, waterproofing or coating the stone with any sealing process will inhibit it's natural ability to evaporate this moisture from inside the stone. This will result in new damage and expansion problems. The War Graves Commission once did some experiments on water proofing compounds and found that silicone treatments actually caused more damage than if the stone was left exposed. The silicone trapped water in the stone, and the frost blew off the surface.
While Saving Graves does not recommend the use any type of sealant, there has recently been developed a number premium quality breathing type sealers which is claimed to not trap moisture within the stone and will not interfere with the natural self cleansing calcification process of stones such as limestone. However these sealants are quite new, expensive and in the opinion of Saving Graves additional testing is required before such a product could be recommended.
The Use of WD-40 as a Gravestone Sealant
There seems to be little information on the effects of the use of WD-40 on a gravestone outside of a comment on the final plans website recommending that on bronze markers after cleaning to spray the memorial with WD-40 to protect the finish. However given the fact that a bronze marker is an entire different ballgame from a stone marker it can safely be assumed that introducing this product to a stone surface would produce quite different results.
The "WD" in WD-40 stands for Water Displacer. It was designed to remove water from mechanical equipment but it was found to have some lubrication ability short term so they went down that road in marketing it. The first concern is that if a person wants to use WD-40 to seal the stone, then one must assume that they will not only clean the stone but also rinse the stone off before applying the WD-40. If that stone is wet when they apply the WD-40 then it will only serve to drive the rinse water and any leftover chemical deeper into the stone and trapping it there.
The second concern is that it has peen proven that WD-40 changes as it evaporates over time into a more "gummy" substance. Given the fact that the WD-40 is applied to the surface of the stone after cleaning as a sealant one has to assume that there is no intent to wash it off. This "gummy" substance on the surface of the stone could create something along the lines of a waterproof seal that could not only trap moisture within the stone, but could hamper the natural breathing process of the stone.
You also must take into account that what goes into making WD-40 is unknown to the general public it's next to impossible to say with 100% assurance what effects to the stone it may or may not have. With that in mind, Saving graves recommends to fall back to "when in doubt, err on the side of caution". Exercise common sense whenever using WD-40.