I found what looks like a good article on the subject of limestone repairs, originally written by Hoffman Architects: Maintenance of Historic Limestone and Marble
Unfortunately, it doesn't cover paint removal. An excerpt reads:
Marble and limestone are relatively soft stones and can be easily scratched and marred. This softness means chemical cleaners, sandblasting and wind-driven grit will take their destructive toll.I imagine if chemical cleaners are going to be harsh, than chemical stripping will take its toll as well. Old House Journal agrees:
As opposed to kiln-dried masonry materials such as brick and architectural terra-cotta, building stones are generally homogeneous in character at the time of a building's construction. However, as the stone is exposed to weathering and environmental pollutants, the surface may become friable, or may develop a protective skin or patina. These outer surfaces are very susceptible to damage by abrasive or improper chemical cleaning.From a message board I found this method for removing paint from limestone:
Try some liquid cooking oil. Rub a coat on and let it sit. It should soften and swell the paint. On another site, I was asked how to remove paint from rocks without harming the moss on them. I wasn't sure, but suggested the cooking oil. It worked, the person was very happy...The oil will soak into the pores of the rock. It will take repeated washing with soap and water over a period of time to remove it. I cringe every time I hear of somebody going to paint brick or stone.Oil seems like it should be mild, but messy. I found a number of sites praising it for removing paint, tar, etc., but haven't come across anything "official" sounding.
From another architectural site I found this, geared toward graffiti removal:
For the removal of the paint from the limestone follow these steps:
- Saturate the wall. This is to prevent driving the paint further into the porous material when solvents are applied. The easiest way to do this is to set up a lawn sprinkler or garden sprayer and direct it onto the wall above and below the graffiti. Let the wall absorb water overnight. There is some possibility that water will saturate all the way through the wall.
- Apply a poultice that is a mud-consistency mix of a solvent (if toluene doesn't work try xylene or methylene chloride) with diatomaceous earth (obtained from a water treatment supplier) or commercial clay. The poultice must be of a consistency that will adhere to the wall.
- Tightly cover the poultice with plastic sheeting taped to the wall and leave for three or four hours. This will allow the solvent to remain in contact with the paint without evaporating.
- Remove the plastic sheeting and allow the poultice to dry. This won't take too long on a sunny day. Once dry, the poultice will begin to crack and fall off. At this point you can use a vegetable (not steel!) brush to remove the admixture. As the poultice dries, so will the wall, and as the water migrates toward the atmosphere it will help carry out loose particles of paint
- Spray the wall with a high-pressure water jet to clean the rough surface. This equipment is available at most rental outlets.
You may need to repeat the process several times.
The cautionary notes that I emphasize strongly are: DO NOT apply the solvent directly to the paint without first saturating the wall. Solvent alone will drive the paint deeper into the limestone. DO NOT use a caustic stripper because it will etch the limestone or brick and leave what, from a distance, will look like an embossed version of the graffiti. DO NOT try to physically remove the paint with brushes, scrapers or high-pressure sprayers alone, because you will remove more stone than paint
All of the solvents are commonly available at hardware and paint supply houses. I would avoid the environmentally friendly paint removers because they are usually non-solvent based material and rely more on acids, which are caustic. They also tend to be expensive and viscous, making them difficult with which to work.
At the Chicago Bungalow Forums, they had a few more suggestions, ranging from steam to power washing. The Chicago Bungalow Association had a PDF on it.
Sigh. It seems like there's definitely some conflicting info out there, with some saying "no" to power washing, chemical strippers, steam, etc., and others saying "yes, this worked for me," trying all of the above and even using sandpaper to smooth the surface. I don't know anymore. I think I'll have to call around some and get a few more opinions from the pros.
Why, why, why do people paint stone and brick?