AkzoNobel 456-21 (21 Series) was a chemically cured two-component, semi-gloss topcoat designed to provide maximum protection from various chemicals, hydraulic fluids, aviation fuels, phosphate ester (Skydrol) fluids and corrosion causing media. This product is no longer manufactured, and AkzoNobel recommend the 456-23 (23 Series) topcoat as an alternative.