This guide on paintwork covers:
Since the 19th century, cement harling was often used in place of traditional lime harling; unfortunately this system is very hard and does not allow the external walls to breathe. This can lead to dampness building up and is most obvious where old lime harling has been ill-advisedly patched with modern cement.
Look for signs to identify areas that may lead to serious problems in the future
The optimal choice depends on the type of paintwork being fixed. Sample areas of paint should first be removed to determine the most appropriate method for each situation. Directions for applying the stripper and cleaning and neutralising the area afterwards must be carefully followed. This needs listed building consent.
Where modern masonry paints have already been used, remove any loose or flaking paint by scraping or wire brushing. Any mould or fungus growth must be removed using a proprietary anti-mould treatment according damaging stonework or flooding the inside of the building. Normally a pressure water wash is only allowed up to 200psi; any higher and consent is required. Make sure that the wall has dried out completely before repainting starts.
Paint should be replaced by limewash. Limewash and lime-harling are specialised products and although less simple to use than modern masonry paint, give a most attractive appearance. You should get specialised advice before using these materials.
Where possible, modern cement render should be removed and replaced with traditional lime harling. Having done that, use the advice below regarding paintwork on traditional mortar. There are several proprietary paint stripping systems available on the market for removing paint from stone such as steam-cleaning, or alternatively a poultice system may be used to remove multiple layers at one application. There maybe supplementary planning guidance on such paint removal [ Needs to be written] .
It is difficult to give advice on such a variety of different external surfaces, but the general principle is never to paint dressed stone surfaces and to use traditional materials like limewash to decorate harled surfaces.
The ingos of doors and windows (see image) should also not be painted.
In Georgian properties, paint should never be used on masonry. Wherever existing paint on masonry needs to be renewed it should be replaced by limewash. Limewash allows walls to breathe and protects the underlying masonry.
Note: Applying limewash to unpainted or un limewashed walls requires planning consent, which will normally not be given.
Sometimes ingos (see picture) have in the past been painted. Such paint should be removed. If the general masonry is unlimewashed then the ingo should also be unlimewashed.