Although the process of removing paint from the exterior of a brick house by means of a lye stripper is in fairly common usage, there are a number of risks involved. The process may involve risks that outweigh the small savings in the cost of cleaning.
Painting of masonry is an old and valued technique. Originating in Europe, the painting of stone and brick was a well-accepted method in the United States by the 19th century. Paint was applied for a variety of reasons.
A well painted dark red with the joints penciled in white was considered esthetically desirable. The paint could mask rough brickwork that was never intended to be seen. Later alternations to the structure could be masked behind a similar coat of paint.
The most important reason for painting, however, was to reduce the permability of such a highly porous material as brick. The paint forms a continuous film that tends to shed water.
For these reasons, it seems obvious that careful consideration should be given before deciding to remove paint from external brickwork. When the paint was original to the structure, removal will expose a surface that was never meant to be seen -- and not designed for direct exposure to the elements.
Moreover, after removal of the paint, another waterproofing or coating would probably be needed. This not only seems unnecessarily costly -- removing one coating only to replace it with another -- but also modern waterproofing compounds generate their own problems.
The simplest and least expensive procedure is to replace or repair the existing paint. Scaling paint can be scraped and brushed with nonferrous brushes. (Steel-bristled brushes can badly damage the surface of the bricks.) If large chips are removed and the surface tends to look patchy, the edges of the surrounding paint film can be feathered by sanding.
If complete removal of paint is deemed necessary, it should be approached with great care. The use of a lye solution, which is highly alkaline, is not without its dangers. The alkali will penetrate deeply into the porous brick and can damage surrounding materials, including painted wood trim.
Lye in the brick can cause efflorescence (formation of salt deposits on the surface) or cryptoflorescence (formation of salt deposits inside the wall). Aside from the unsightly appearance of these salt deposits, the constant process of crystalization and recrystalization can seriously affect the brick and eventually cause spalling and disintegration.
These dangerous side effects are one reason why cleaning with steam and caustic soda (lye) is generally considered undesirable today.
When any chemicals are used in stripping paint from brickwork, whether it is lye or other chemical strippers, removal of any residue is critical. Thorough rinsing with water is the least dangerous method. Too much water can, of course, soak the wall and damage interior finishings, as well as cause deterioration of embedded iron ties and wooden structural members.
To neutralize alkaline residue or remove efflorescence with strong acids such as hydrochloric (muriatic) acid is quite risky. (not only only is there a chance that the acid will "burn" the brick, but also damage to the mortar joints is almost unavoidable. The soft pointing martar, especially the lime mortar used in older structures, is highly vulnerable to acidic solutions. Disintegration of the pointing mortar will require immediate repair.
Once the masonry is thoroughly rinsed with water, the wall should be left to dry for a minimum of two weeks to be sure that no further efflorescence occurs.
Modern waterproofers have their own problems. The often-recommended silicones have only a limited lifespan, about three to five years, thus requiring frequent reapplication, a costly procedure. Coatings such as acrylics give the surface a sheen, while totally sealing the masonry. This increases the likelihood of salt build-up behind the coating.
If paint is to be removed, the safest course is to use one of the specially formulated paint removers designed for use with masonry. These will have a number of addittives that increase the surface activity of the remover while avoiding too deep penetration into the brick and etching of adjoining materials.
Moreover, if used in the form of a poultice, the chance of too deep penetration is further reduced.