Chemist's Commentary on Treatment Approach
Cleaning: AmberClean SC19
The use of oil-painted masonry, along with other cementitious, and ceramic (terra cotta) materials on the building facade set up an interesting set of requirements and restrictions or limitations for cleaning materials that might be appropriate in this context.
Usually highly alkaline conditions promote the deterioration of oil binding materials like linseed oil in exterior paints through de-esterification or "saponification" type hydrolytic reactions. Alkaline effluent salts from mortars can degrade these kinds of paints, but so too, can very alkaline or strong detergent cleaning systems. The apparent use in the past of sodium stearate/palmitate-type detergent such as Ivory soap would have exacerbated the breakdown of the remaining original oil paint on the masonry, and, ironically, would have made the recovered or "cleaned" surface less saturated (and looking worse), than before cleaning, as more paint binding material was lost. This seems to have been the cleaning history of the exterior in a surface that looks whitish, blanched from paint deterioration, and was therefore to be cleaned again to address the "problem" of a whitish, occluded surface, only to have once again exacerbated this aspect of the surface, and not alleviate it.
The high amount of ionic material in these kinds of traditional detergent or soap mixtures would also be difficult to rinse from porous metal or metal oxide (e.g. ceramic/brick) surfaces. Residual salts would, of course, contribute to salt fretting on the porous masonry and terra cotta. But additionally, the high HLB number (surfactant "strength") for sodium stearate/palmitate soaps aren't required in this instance: the building, as it turns out, is only lightly soiled, and even modestly strong (lower HLB) detergents would more than suffice for cleaning purposes in this instance.
The oil-painted masonry surface, the pointing mortars, and the terra cotta elements define in this context the appropriate pH, and other general features we need to think about or "shop for" in an aqueous cleaning system. A near neutral pH, for instance, would not promote the dissolution of the pointing mortars, and certainly not further the saponification of remaining original oil paint. Additionally, non-ionic surfactants could and would be useful in a situation where we would want to avoid or replace ionic surfactants like traditional soaps, to avoid the build-up of salt forming or containing residues in and on the porous surfaces.
AmberClean SC19 (St Ghobain) is a commercially available detergent system that meets these requirements: near neutral pH (7.5), non-ionic surfactant system, and (in this case) ultra low conductivity (that is, less than 100ppm total ion concentration). Its chief industrial use is found in the semi-conductor industry, where both cleaning and clearance of cleaning materials are critical (St Ghobain).
AmberClean SC19 can be applied with a simple pump type sprayer; is water rinsible or reducible; and can be directly remediated into ground or sewer systems (it is biodegradable) along with solubilized soiling from the building under low-pressure water rinse.
Coating: Avalure 315
Because of its polyacidic nature, poly acrylic acid (PAA) can bond to metal and metal oxide surfaces through both ionic (acid-base) and polar (hydrogen bond) interactions (Sugama 1984). It has found wide commercial use in as far ranging applications as cosmetic, coating, and adhesive industries. PAA either as a homopolymer, or in blends with other polymers, or as a graft co-polymer with other polymers has been used in applications as both a suspension aid for metal or metal oxides (e.g. pigments in paint and coating systems), as well as an adhesion promoter to surfaces that contain these materials because of its ability to interact with these materials so strongly and intimately.
In the present context, PAA or PAA-modified materials like Avalure 315 (Avalure 315 is a co-polymer of acrylic acid and acrylate esters) should therefore adhere well to surfaces like ceramic, masonry, cementitious, plaster, and highly pigmented materials that contain metal or metal oxides (iron oxide pigments). By delivering the Avalure 315 in a polar solvent solution (ethanol) we've essentially favored the free-acid form of the PAA present in the polymer. Predominantly, therefore, the adhesion to brick and paint surfaces will initially be through hydrogen bonding with alumino-silicate and other metal oxide materials (e.g. pigments) that are hydrated or contain hydroxyl groups. This strong interaction allows us to create the visual or aesthetic properties we want to recover from these damaged surfaces: saturation. By saturating the surface of pigments freed from their original binding material (oil) by weathering and prior cleaning, for instance, we can recover the darker, richer and more saturated color of the remaining original surface paint.
It's important to note that the Avalure 315 isn't applied as a continuous film, but only in a manner and at a concentration sufficient to only coat the individual particles that are at or near the application surface. We've specifically chosen a material like Avalure 315 to perform in a "gate" or "immersion resistant" (He, et.al. 2007) manner. PAA containing materials can be swollen with liquid water, closing off fine pores, and can, in effect, act like a "gate" to preclude liquid water from penetrating fine pore structures. On drying, these materials lose moisture, the swelling subsides, and moisture vapor can be transmitted through these treated surfaces again. Avalure 315 is a "smart" coating material repelling liquid water and forcing it to run off treated surfaces, while allowing the masonry in this case to breath or transmit water vapor as the building wants to gain or lose moisture overall. This is a decidedly different strategy from silicone coatings that have been routinely applied to these kinds of surfaces in the past (and which worked by creating closed or continuous coatings that were very hydrophobic in character).
Aging studies have demonstrated that PAA containing films like Avalure 315 are inherently remarkably resistant to both thermal and photo-oxidative deterioration. Mechanically, Wu, et.al. (1997) have reported that the thermal stability of PAA is actually increased with bonding to metal and metal oxides. Moreover, compressive strength appears to be optimized with bonding to these materials as well. PAA graft co-polymers have actually been reported to increase the photo-oxidation stability in usually vulnerable hydrocarbon polymer systems like isotactic polypropylenes (Bhattacharya, et. al. 2006) when bound to these materials. This latter observation is especially important in this use context because the applied Avalure 315 film may actually provide an addition bit of stabilization to the remaining oil binding material in the original brick paint. The Avalure 315 is applied "neat"; that is, without additional UV or free-radical stabilizers that would normally be used in exterior coatings.
Avalure 315 was applied from an alcoholic solution, as noted above. But it retains its water solubility under favorable conditions: at slightly alkaline pH (8-9) and with the addition of even mild chelating agents like citrate or NTA, the Avalure can be washed from the building surface and remediated directly into ground water or sewer systems. This ease of reversibility allows us to use the Avalure 315 as a soil-release, re-treatable coating material that can facilitate future cleaning and re-coating as needed.
In summary: Avalure 315 as a coating material in this context provides four essential properties for us. 1) It promotes the original paint color, saturation and appearance. 2) It acts to allow moisture gain and loss from the building, while still repelling liquid water. 3) It is a tough, long-lasting material that resists thermal and photo-oxidation. And 4) It is a reversible and easily remediated exterior coating material.
Color Restoration: Carboset AE 960
As odd as it sounds, "in-painting" or re-introducing color to those areas of painted brick and terra cotta that are so badly damaged by staining or weathering that cleaning or re-coating alone cannot remedy, seems to be fairer in the overall goal of re-presenting the building exterior in the best possible aesthetic light. Cleaning by itself will certainly not suffice; nor will the re-introduction of a coating material to re-saturate the degraded paint surface in all instances. But in this case a more "usual" masonry coating material, lightly pigmented, might suffice.
Carboset AE-960 is a high solids, water based, 100% acrylic emulsion specifically developed for elastomeric vertical masonry coatings (B.F. Goodrich). It can be diluted with water to in effect drop the amount of solids to produce a "matte" color-containing binding system. Among its other attributes in this application are: excellent resistance to dirt pick-up; efflorescence and salt formation resistance; good weathering; water resistance, and good adhesion to cementitious materials (B.F. Goodrich). As a PAA containing polymer dispersion, the Carboset acrylic resin can be applied as an aqueous solution, but also removed under desirable aqueous conditions (slightly alkaline pH of 8-9) and with a mild or weak chelating material (e.g. citrate or NTA).
The pigments of choice in this application would be the most stable: iron oxides; earth colors, titanium white, etc.