active ingredient

In the context of painting and coatings, the term "active ingredient" refers to the component in a product—such as a paint, primer, or chemical treatment—that performs the primary intended function. This could include:

• Pigments that provide colour and opacity
• Binders/resins that form the film and adhere the paint to the surface
• Biocides or fungicides that prevent mould or mildew growth
• Rust inhibitors in metal primers
• Solvents (in some cases) that aid in application and drying

In general, the active ingredient is the part of the formulation that delivers the key performance benefit, as opposed to inert or inactive ingredients that serve as carriers or fillers.

For example, in an anti-mould paint, the active ingredient would be the fungicide that prevents mould growth.