You haven't mentioned a product but for pitched, corrugated asbestos roofs we usually recommend Eagle DoubleCoat Pro-Reflect which is a liquid elastomeric waterproofing membrane, reinforced with microfibers and high aluminium content. Used for waterproofing most types of roof surfaces to include galvanised steel, iron zinc, asbestos, cement, roofing felt, asphalt, bituminous roof coatings and anticorrosion to exposed metal structures such as storage tanks, pipes and cladding and it is highly recommended for industrial units and warehouses.

After suitable cleaning and preparation (observe local regulations), apply 1 coat of Eagle Quick Drying Bitumen Primer, followed by Eagle DoubleCoat Pro-Reflect.

Based on 18m² (20ftx10ft) you would require 1 x 5lt Eagle Quick Drying Bitumen Primer + 1 x 25lt Eagle DoubleCoat Pro-Reflect.