Bubbling, flaking and peeling of paint used on masonry have a number of causes. The most common are the chemical nature of the masonry itself and the use of the wrong type of paint.
Moisture in the masonry mix, seepage of moisture through porous masonry from natural sources and a condition called efflorescence are other reasons for the paint failures.
But there is a way to keep paint on masonry walls and ceilings: Proper surface preparation and the use of the right products can almost guarantee a long-lasting paint job on any masonry surface.
Man-made forms of masonry change from an easily formed plastic state to rock-like hardness through a chemical change. Once cured, the surface may be either acidic or alkaline, depending on the type of masonry used.
To avoid paint failure caused by pH extremes on the surface of newly cured concrete, wait at least 30 days before painting. The pH problem will fade as the masonry ages.
Moisture is the biggest culprit in paint failure, whether the surface is masonry or wood. But masonry presents its own special set of problems.
Masonry is porous, so water can penetrate and seep through the entire structure. As it penetrates through to a painted area, water pushes the paint off. A wall that leaks after a rainstorm is a common example of this problem.
In many cases, says Richard Barako, lab safety coordinator at United Gilsonite, application of a masonry waterproofing paint will solve the problem of seepage.
Of course, if you have walls that leak, you should correct the cause of the leaking first. Look for improper grading that carries runoff toward the foundation rather than away, damaged gutters and downspouts and malfunctioning foundation drainage systems.
Masonry surfaces frequently develop a condition called efflorescence. This condition is typified by chalky white stains on brick or concrete block walls. Efflorescence results when soluble salts present in the masonry mixture are carried to the surface by water. The water evaporates, leaving the salts behind in the form of white stains. No paint, not even one specially formulated for masonry, will adhere to efflorescence. To remove it, you must "etch" the surface.
Masonry is etched by using muriatic acid or an etching product that is milder than muriatic acid but adequate for the job. After mixing the acid according to directions, apply it with a brush to be sure it gets in all the pores.
The acid solution will foam briefly. When the foaming stops, the salt is neutralized. The surface should then be thoroughly rinsed. Do not let the acid dry on the surface or it could interfere with the adhesion of the waterproofing material or other coatings.
Etching should also be performed on masonry floors before painting if the surface is very smoothly troweled. It performs the same function as sanding a high-gloss wood surface before painting.
To ensure a successful paint job, make sure the surface is free from dirt, oil and grease. If there is any doubt about the latter, wash down the surface with tri-sodium phosphate or a detergent and follow with a thorough rinsing.
To choose a product that is right for the masonry surface you are painting, decide first how the paint must perform.
If moisture is a problem, choose a product that is labeled "masonry waterproofer." Ready-mixed varieties do the best job and solvent-based products outperform latex formulations.
Powdered products that you mix with water are an economical choice and perform well, although not as well as the solvent-based products. Cement-based mixes, like cement paint, are excellent choices if you wish to decorate masonry walls where water is not much of a problem.
The masonry waterproofers, the powdered product and the cement paint are all made with portland cement. By using a product made with the same material as surfaces on which it will be used, you increase the chance of good adhesion.
Ordinarily, latex or oil-based wall paints can be used on masonry walls where moisture is not a problem and is not likely to be in the future. Masonry floors and patios require special care and special products. Often, masonry floors suffer from a dusting condition. Paint will not adhere properly because the floor will continue to dust, taking the paint with it.
First you must cure the dusting problem. In extreme cases, the floor should first be etched. Then, the application of a solvent-based clear masonry sealer will eliminate the dusting problem, giving you a sound surface for paint.
Floors must also endure foot traffic and repeated washings, so use a product specially formulated for use on masonry floors. There is a latex concrete floor paint which works equally well outside (on patios) or inside (or on garage floors).
Barako says that there are masonry areas around most homes that could benefit from proper application of the right product. These include walls and floors in garages, exposed foundations, retaining walls, patios, painted brick walls and swimming pools.