REDECORATION OF STUCCO
Portland cement plaster, often called stucco, can be redecorated in one of several ways. An additional coat of manufactured or job-mixed stucco may be spread directly over the original surface, or a bonding agent followed by pplication of an additional coat of stucco may be applied, or paint may be brushed, rolled or sprayed onto the surface. Preparation of the surface of original stucco should be essentially identical for each of the three alternative procedures described above. If, at one time, the original stucco had been treated with a waterproofing compound, additional preparatory procedure may be required, depending upon the type of waterproofing agent which had been used. If the wall or soffit had been treated with sodium silicate, the contractor may proceed in the usual manner, because sodium silicate is compatible with portland cement plaster.
If the waterproofing compound was asphalt, or one of the stearates such as calcium or aluminum stearate, or was carried in a petroleum solvent or was a petroleum derivative, such as silicone, extra precaution regarding testing and preparation of the existing surface is especially critical.
Petroleum-based and water-based systems are not compatible under normal circumstances, therefore, in those special cases, the contractor must attempt to overcome resistance of the wall or ceiling to application of water-based stucco, bonding agent or paint. Most, if not all bonding agents are dissolved or suspended in a water vehicle. Water will not satisfactorily wet a wall that has oil, stearates or certain other, but not all, water-resistant compounds on or in the surface of existing stucco. If the wall cannot be wet satisfactorily, bond may not be achieved.
If sodium silicate had been employed to treat the surface of original stucco, the contractor may proceed in the manner which I will describe for untreated stucco. The reason for exclusion of sodium silicate from the category requiring special treatment is that sodium silicate is compatible with concrete, portland cement plaster and masonry. Even though the chemical may be employed to reduce water absorption of masonry, plaster or other base, a coat of cement plaster may be safely applied over a sodium silicate-treated substrate.
The following suggestions concerning preparation of plaster walls and soffits for redecoration are offered for consideration by architects and contractors.
If the existing surface has not been treated with oil, silicone, stearates or other materials with similar characteristics, the wall or soffit may be cleaned and decorated. It is recommended that existing walls and ceilings be hydroblasted with a jet stream of water to remove dirt and debris which clings to all types of walls. Aerial debris can and sometimes does act as a parting agent, to prevent desired bond of the new coat to the old. After hydroblasting, the wall or soffit should be allowed to drain and lose excess water to evaporation before application of a new coat of stucco or other decoration.
If a wall has been treated with a waterproofing compound that resists wetting, as previously described, it is important that careful study, testing and preparation of the substrate be accomplished before application of water-based coating is attempted. It is even more important to recognize that it may not be possible to achieve satisfactory bond after vigorous scrubbing with a powerful cleanser, over certain water-resistant treatments. If scrubbing a wall with a powerful cleanser does not promote good bond of test patches, the substrate must be abraded by sandblasting or hydroblasting, or the new decorative coating must be attached physically, as with installation of metal lath secured to the base, which provides positive attachment to the substrate.
The following suggestions, other than installation of lath, concerning preparation of treated plaster surfaces to receive a new decorative coating, may not develop compatibility with water-based coating, if certain special water-proofing agents had been previously applied to the original base. After a wall is treated in accordance with the following procedure, the surface must be tested for bonding capability by application of test patches of the coating intended as decoration. Test patches should remain in place for a sufficiently long period, sometimes about a week, to provide reasonable evaluation of bond between test patches and the substrate.
To prepare a previously waterproofed wall for an overlay of stucco, bonding agent or other coating, the wall must be scrubbed with a powerful cleanser such as trisodium phosphate. Prior to scrubbing, if a new wall or soffit that was newly waterproofed is involved, it would be advisable that the surface be exposed to sunlight, heat and weather for at least a month, to improve possibility of good bond. The benefit of this described exposure is that some treatments, such as silicone and stearates, gradually alter when subjected to exposure to the described conditions.
Prior to start of preparatory work for redecoration, tools and equipment essential or helpful to preparation should be assembled. A list of recommended items follows.
MATERIALS AND EQUIPMENT
Trisodium phosphate (TSP). Available at most paint and hardware stores.
Rubber or plastic bucket, to mix and hold the solution of trisodium phosphate.
Long-handled stiff fiber brush, for scrubbing walls.
Water hose, with good pressure.
Jet nozzle, to be attached to the hose to ncrease pressure of the stream f water directed gainst the wall or soffit.
Safety equipment, including:
rubber boots, and
plastic apron, or raincoat, to protect clothing.
At the start of the operation, the wall or soffit should be flushed vigorously and thoroughly with a powerful jet stream of water, to remove all surface dirt. Washing should be done in accordance with suggestions provided in my technical article entitled "CLEANING AND CARE OF STUCCO WALLS".
Trisodium phosphate should be mixed in a cold water solution in accordance with directions on the container or instructions from the paint store where the TSP was purchased. While the wall is still damp from being water-blasted, the entire wall surface should be scrubbed with the solution of trisodium phosphate. Goggles, rubber gloves and other safety equipment must be worn to protect eyes, hands and skin while using the solution. The user must bear in mind the fact that TSP is a strong cleanser, often used to remove dried paint and other coatings.
The surface of stucco is to be scrubbed thoroughly with the fiber brush and TSP solution, to work the trisodium phosphate deeply into the surface and achieve maximum effectiveness from the cleanser.
At least one-half hour exposure of trisodium phosphate on the wall should be arranged. Additional time would not be harmful and could be beneficial. After adequate exposure time of TSP on the surface to be cleansed, the wall or soffit should be washed effectively with a jet stream of water, to remove all traces of trisodium phosphate and affected waterproofing. This second period of hydroblasting is especially important, since any remaining TSP could be adverse to new paint or bonding agent that is to be applied later.
A second treatment of TSP dissolved in water, just like the first, may be desirable. Diluted trisodium phosphate will not be harmful to shrubbery, concrete walks or wood.
The wall or ceiling which has been cleansed in this manner should be allowed to lose much of the excess water that has been absorbed during the scrubbing operation, before start of application of a bonding agent, stucco or paint, because a saturated wall is not receptive to most coatings.
After the wall or ceiling has been thoroughly scrubbed as described, and prior to application of a new coat of stucco, paint or other decorative treatment, the substrate must be tested to determine acceptance, with good bond, of the new coating. After excess water has drained and evaporated from the wall, three or more test patches of the bonding agent, stucco or paint that is proposed to be used should be applied to different locations on the surface. It is desirable that test patches be about one square foot in area and spaced far enough apart to properly ascertain suitability of the washed surface.
Test patches should be properly cured, then allowed to dry and become firm, and left in place for at least several days for optimum bond to develop between patches and the substrate. After a reasonable period of time has elapsed, attempts should be made to peel patches from the wall. If delamination does not occur, it is reasonable to assume that a complete coating of the same material would bond to the wall or soffit.