AB Note: Thanks to one of our readers who made a very important point regarding the mixing of water and lye. I'll post it verbatim: "This article on your site instructs readers to: 'put a quantity of the caustic and slowly add water, stirring gently until the caustic is dissolved.' This is very dangerous. Lye (mentioned earlier in the article) should be added to the water and not vice-versa. If water is added to lye it can react and explode or splash."

There are three options open to the owner who wishes to remove exterior paint: (1) sand-blasting; (2) hand-scraping; (3) chemical solvents. Of the three, only the third presents a treatment that will leave the original surface intact, and is relatively easy. This article will deal with this third method, limited to masonry buildings covered with an oil base paint.

Since oil base paint is acidic in nature, it will be chemcally broken down by an oxidizer or caustic base. The base used in this instance is caustic soda (lye), purchased at a chemical supply house in 100 pound kegs, flake form at about 20 cents per pound. For smaller applications, household lye sold in supermarkets work just as well. It is a good idea before buying the larger quantity of lye to test the surface with a small batch of remover made up with household lye. The test sample is prepared in the same manner as the larger quantity.

Remember that working with any caustic requires a great deal of care. Rubber gloves with long wrists, hat, plastic face shield, long-sleeve shirt, etc., are musts. A rubberized, hooded slicker is ideal. If the solution gets on the skin, it should be washed immediately with plenty of water. It can also be neutralized with vinegar.

The idea behind this entire procedure is to stick the caustic to the painted surfaces, because a plain solution of caustic and water will not adhere to a vertical wall. A thickener is needed. This can be a number of products including Cab-O-Sil (fumed silica), fluffed calcium stearate or plain cornstarch. I used cornstarch and found it quite satisfactory – as well as inexpensive.

To begin, two non-metallic containers are needed. I prefer 5-gallon, plastic, joint-compound buckets. In one, put a quantity of the caustic and slowly add water, stirring gently until the caustic is dissolved. In the other bucket, put cold water and add the thickener. When using cornstarch, add until the water turns a mily white in color and stir until all lumps are dissolved. Then add the thickened solution to the caustic which should immediately become thick and syrupy. The strength of the remover varies directly with the amount of caustic used. I usually make about a gallon of each solution using perhaps a pound of caustic soda. The coverage will depend upon the painted surface to be stripped. Since there are so many variables, much individual experimentation is required.

A nylon-bristled paint brush will not be affected by the caustic. Whenever possible, I use stiff, discarded brushes, because the remover will clean them like new, thus yielding a highly desirable bonus!

Usually, immediately upon application, the paint will start to break down. When there are many layers, only the top-most layers will come off at first. But remember, as in all types of paint removing, patience is the watchword. The longer the remover is on, the more paint it will remove.

Once the surface is coated, it should be kept moist. Moisten by gently sprinkling with just enough water to keep the remove intact without causing it to run. After about an hour, test the surface with a hard jet stream from a garden hose.

If the paint washes off down to the original surface, it is ready. If not, wait longer before testing again. In situations where there are many coats of paint, it is possible that the remover will not cut all the way through. In this case, the remove must be washed off and the entire process repeated. I once let a coat of remover remain for a week, and the paint was not completely removed.

Here, however, is where the experimentation comes in, vary in the strength of the solution. A handy tool is a pump which will increase the water pressure used in connection with a wand at the end of a hose, a knife-like jet of water can be produced that will lift the softened paint much more readily than a garden hose. These pumps and hose attachments are used in building-cleaning and probably can be rented.

Once the paint is removed, the surface should be washed down with acid to neutralize any remaining caustic. Hydrochloric will remove dirt. Extreme care must be exercised whenever working with any of these acids as they will harm human tissue, plants, trees, parked cars, etc., as will the paint remover. These acids must also be thoroughly rinsed off with water.

When working on the exterior of a building, extreme care must be taken to properly protect adjoining properties. Check first with abutting owners and mask off any surfaces that that might be contacted by any of the chemicals or the water used in their removal. Sheet-plastic works well as a protective covering, and is readily available.

There is also a possibility that a permit may be required from the local building inspector, especially if the work is to be done on the front of the house.

The removed paint usually gathers at the base of the building and after the water runs off, it can be shoveled into a water container. Since the paint does not dissolve in water, I do not think there is any environmental damage from the water that runs off into storm drains.

Once a brick building has been cleaned, there is usually a tendency for the bricks to effloresce. This can be removed with muriatic acid and will sometimes disappear when a sealer is applied. The sealing should be done a week or two after the cleaning when the bricks are thoroughly dry. In older buildings, the water jets may rake loose mortar from the joints and make pointing necessary. Pointing is probably advisable in any case and a clean surface improves the adhesion of the pointing mortar.

My own home was cleaned and treated in the way I have descrived with highly satisfactory results. I know of no commercially available process or material that removes paint from exterior masonry economically. With the current trend in architectural preservation and restoration, perhaps a better method will be available soon. Until then, the caustic and water approach will have to do.