Work on stone in period houses and other old buildings should be kept to a minimum. The history of the stone is represented by its dirt; this only needs to be removed if it threatens the long term survival of the stone. Stone can cope with moderate failures of the pointing unless it is in an exposed position when water can penetrate, freeze and cause damage.
Stone and cast stone containing calcareous material such as marble or limestone may sometimes be cleaned with an alkaline pre-wash/acid afterwash chemical cleaning system. If no calcareous materials are present, acidic cleaners may be used.
In either case, dark particulate staining, for example soot, may be persistent and require experimentation with other cleaning methods. Some micro-abrasive cleaning techniques used under very controlled circumstances by skilled cleaning personnel can be appropriate for removing tenacious soiling. Ordinary sand blasting or wet grit blasting can seriously damage the surface of soft and cast stone and should not be used.
Stone walls and architectural details were usually constructed with lime mortar, but from the late 19th century, stone units were generally bedded and pointed with mortars composed of cement, lime, and sand. It is however better to use lime and sand only on softer stone such as sandstone and cast stone.
To remove old pointing takes great skill and care. A hand-held grinder fitted with a thin diamond blade can be used to score the centre of a joint. The rest of the mortar is then removed with a hand chisel. If this method is not done carefully, however, wandering of the blade can widen or alter joints and cause significant damage to the stone. Care must be taken to prevent damage from the over-cutting of vertical joints by stopping the blade well short of adjacent blocks. The use of small power chisels, such as those used to tool stone, can also work well for mortar removal, but even this method can cause chipping to the edges if it is not done carefully.
When repointing or replacing lime mortar, use a 3:1 sand/lime mix. When the original mortar includes cement, a 6:1:1 sand/cement/lime mortar is generally appropriate.
When repointing any masonry, it is important to match both the character and colour of the sand and colour of the cement matrix in the original mortar. Adjust the colour of lime mortar with different colours of sand. In cement mixes, the colour can often be adjusted by using combinations of white, 'light', and grey portland cement in the mortar.
Before beginning a repair, experiment with different recipes or else ask for an expert opinion; this may include chemical analysis.
Painting stonework is not a cheap alternative to proper repair. It usually causes rather than solves decay problems. Stone walls must be able to breathe; most paints, especially those which are oil-based, block the pores and trap moisture inside. Painted stonework must still be washed and repainted every few years.
If soft or flaky stonework is painted, the surface will soon start to peel. Ideally, a painted building should be carefully stripped and the original stone finish restored.
If paint cannot be removed, use a special micro-porous masonry paint, preferably in a muted colour. Limewash will not adhere to masonry paint but can be used where the paint has peeled while you wait for the rest of the paint to decay.
If having removed the paint, the stone is in a bad state, perhaps with cement repairs, there are some stone effect paints which give a good impression of stone - they contain a sand and have a sandstone colour.
If you still want the stone painted white, then you can just remove any flaking paint, or use a stripper, rinse and then repaint.
Avoid cement as a filler for minor repairs in soft stone such as sandstone; it would be best to use something softer like lime and sand. Or even a decorators filler if you are repainting.
If you want to remove paint from smooth stonework such as window cills unpainted, careful use of a scraper will take a lot of flaking paint off. You can use chemical stripper on the remainder - the poultice type is best, covering it with polythene for a few days, then scraping it off and rinsing very thoroughly.