Graffiti Removal from Soft Surfaces
Ed Crocker

I recently received a call from a victim of graffiti artists. Two adjacent walls of a historic building in Las Vegas had been decorated on a Saturday night and the owner was understandably concerned about removing the paint without damaging the underlying historic materials, limestone masonry in one case, painted brick in the other. The painted brick is easy:  apply any solvent-based chemical stripper, brush lightly with nylon bristles, rinse, and repeat, let dry and repaint. The limestone masonry is another matter.

For one thing, limestone is a very porous material that absorbs both the paint and any chemical strippers that may be applied. The other problem is in dealing with a rough, unfinished surface that cannot be scraped because it is both soft and irregular. The prescription for removal is a bit complex but should, if patience is applied, result in a clean, ghost-free surface.

The first step is analytical; find out what kind of paint was used. In the Las Vegas case, in the absence of a discarded can, I called the local hardware stores and narrowed down the likely product. I found that there was about a 75% probability that the paint was enamel intended for metals. A search of manufacturer websites and a few follow-up phone calls provided the most likely chemical removers. In this case a solvent was called for, probably toluene.

For the removal of the paint from the limestone, follow these steps:

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.