Code of practice for painting asbestos cement building products
1965 Edition

Recommended paints for asbestos cement surfaces include alkali-resistant porous paints for exterior applications, which facilitate breathability and reduce blistering and peeling caused by moisture and alkali reactions. For internal surfaces such as the water-contact areas of gutters and downpipes, a bitumen-based waterproof coating should be applied prior to installation to prevent water damage. For back painting, alkali-resistant primers conforming to IS:109-1950, bitumen paints per IS:158-1950, or colorless waterproofers are advised to protect against moisture ingress. Surface preparation should ensure thorough drying, roughening of glazed areas, and the application of at least two coats of primer to manage suction variability and alkalinity. This approach ensures the durability and effectiveness of the paint system on asbestos cement substrates.

Preparation involves ensuring that asbestos cement sheets are completely dry, typically by exposing both sides to air for 7 to 10 days to eliminate moisture. If moisture ingress from behind is anticipated, back painting with an alkali-resistant primer (IS:109-1950), bitumen paint (IS:158-1950), or colorless waterproofer should be applied prior to fixing. Surface roughening is necessary especially on glazed patches to enhance paint adhesion, followed by the application of a minimum of two primer coats to address surface suction variability. The use of aggressive chemicals like hydrochloric acid or zinc sulphate is prohibited as they can damage the substrate and impair paint adhesion. For gutters and downpipes, an internal waterproof bitumen coating is applied before erection, while the exterior is coated with alkali-resistant porous paint to permit ventilation.

Back painting serves to prevent moisture penetration from the rear side of asbestos cement sheets, particularly when an impervious paint system is used on the front face. This protective measure safeguards the sheet edges and back from moisture and alkali-induced deterioration, enhancing the lifespan of the product. The process involves applying at least one coat of an alkali-resistant primer, bitumen paint, or colorless waterproofer to the back and edges. Care must be taken to avoid contamination of the front surface with back paint materials to prevent discoloration or drying problems. Ideally, back painting is executed after the front face painting but may be carried out before sheet installation if necessary. If existing paint films are intact, one or two finishing coats can be applied accordingly.

The standard prohibits the use of burning or blow lamps to dry or remove old paint due to the risk of cracking. Similarly, the use of hydrochloric acid or zinc sulphate is forbidden as these chemicals cause brittleness and leave powdery residues that impair paint adhesion. Fungicidal treatment involves thoroughly scraping and rubbing the affected surface with glass wool and sandpaper, followed by washing with clean water and drying. Subsequently, two coats of fungicidal wash are applied, allowing sufficient drying time between coats. Painting is done directly over the fungicidal wash without washing it off. After 24 hours, the surface should be brushed gently to remove dust. These steps ensure effective fungus eradication without compromising the integrity or finish of the asbestos cement surface.

Yes, the code specifically advises against the use of hydrochloric acid and zinc sulphate for cleaning or neutralizing asbestos cement surfaces before painting. These substances can make the sheets brittle and leave powdery deposits that hinder paint adhesion. Instead, it recommends ensuring that sheets are thoroughly dry before painting, applying alkali-resistant primers or bitumen paints for back painting when moisture ingress is a concern, and roughening glazed patches followed by at least two primer coats to regulate surface suction and protect against alkalinity. This approach avoids substrate damage and ensures a durable, well-adhered paint finish.