Sign legibility can be restored or improved by general cleaning and removal of foreign substances from the sign face. Sign cleaning products are commercially available for use in removing common soil and severe contaminants such as paint and adhesives. In the majority of cases the required cleaning products can be obtained from local hardware stores. Strong solvents and incomplete removal of the contaminant can damage the sign reflectivity. Strong solvents should be trial-tested prior to application on the sign. The following steps can be used to clean signs of common soil and some contaminants [47]:
• General sign cleaning: Mild, nonabrasive cleaners and detergents suitable for painted or enameled surfaces are recommended for removal of common soil. Cleaners should be free of strong aromatic solvents or alcohols and be chemically neutral (pH of 6 to 8 is recommended).
• Pollen and fungus can be removed by washing the surface with 3 to 5 percent solutions of laundry bleach (sodium hypochlorite). This should be followed with detergent wash and a clear water rinse.
• Lipstick, crayon, tar, oil, bituminous materials, and some oil-based paints can often be removed with mild solvents such as mineral spirits (toluene), kerosene, heptane, or naphtha. Wipe the contaminated area lightly with a soft cloth saturated with the solvent. Continue wiping lightly until the contaminant is removed. If this does not work, then try the next step.
• Wipe the contaminated area with a soft cloth moistened with lacquer thinner. Continue wiping lightly until the contaminant is removed.
• A nighttime reflectivity check or a nighttime visual inspection of all signs from which contaminants have been removed should be conducted.
