The deterioration of natural stone surfaces due to damage caused by the use of inappropriate repair materials can be rectified in a few ways. There are a few options which really depend on the status of the building being repaired. The main repair method would be indenting the stone with a new matching stone. Another option where a minor cosmetic repair is required would be carrying out a mortar repair to fill out the face and provide a nice aesthetic finish. My personal preference for this is using St.One stone repair by St Astier based in France. This was formerly known as Lithomex.
St.One has a high vapour permeability much like most natural hydraulic lime mixes allowing the transport of water vapour through the material and a relatively low strength allowing its use on most natural stones and even soft-fired hand made bricks. The stone repair material should always be weaker than the substrate material. Its ability to be colour matched and shaped and tooled like stone makes it a superb option.
Before carrying out any repairs, the first thing to look at is the condition of the substrate and the depth of the repair. As a rule of thumb most repairs I carry out are less than 50mm in depth back to a good solid base substrate. Any more than that would generally be a new stone indent. There are exceptions to this rule based mainly on cost of repair for highly detailed stonework with isolated damage.These repairs require further preparation and the addition of stainless armatures to support the repair material. For this guide I will be focusing on mainly the simple cosmetic repair.
The first step to take its cutting back any loose and friable material from the damaged stone. The best method for this is using a scutch comb tool to cut back, this takes away all the damaged and soft surface and leaves a good scored surface which is ideal for the adhesion of the new mortar repair giving mechanical grip. This must be done to the entire surface to ensure a consistent key. You must also ensure that if you are only doing a part stone repair the edge must be cut back square to a minimum of 5mm in depth. You should not feather out these materials as the edge will fail.
Ensure all the prepared areas are thoroughly cleaned and dust free prior to application of the new mortar repair. Always pre- dampen the background to control the suction. If the material is applied to a dry background it will suck the moisture from the mortar causing failure at the contact surface. St.One stone repair should only be applied at a maximum of 40mm in one pass according to the manufacturer's specification. From experience even this is a bit of a push and will cause a bit of slumping. I tend to stick with 25mm roughly per layer. If you need to do two layers you must allow the fist to cure and stiffen for roughly 24 hours before applying the second layer. When applying just one layer it is good practice to apply as a two stage process. One very thin tight pass ensuring you press firmly and the bond is good. Then apply straight over the top with the full coating brought out to roughly 2mm beyond your finished face to allow for scraping back. Mortar will stick to mortar better ensuring you have a great bond throughout the repair.
Cover the new stone repair mortar and protect from the weather for anything between 5-24 hours depending on the temperature until the material has stiffened sufficiently to allow rubbing back. St.One stone repair should not be left finished of a trowel, you should always rub back using a render scratch float for flat finishes and any form of sharp edge for shaped surfaces. This also gives the correct textural finish and colour reproduction.
Once the material has been scraped back the tooling can be added if required. This can be carried out using standard stonemasonry chisels or custom made tools. Care and time should be taken to match the existing masonry detailing. You can carry out tooling for up to a week after applying the material, however it is easier while the material is relatively fresh.
It is always best to speak with an expert when looking at carrying out or specifying a repair strategy for stone repair. a great deal of variables must be looked at to ensure this is the correct method to use. I am always happy to discuss any projects and ensure the correct method is used.