Binder is the main ingredient of paints. Binders are polymers (resins) forming a continuous film on the substrate surface. Binders are responsible for good adhesion of the coating to the substrate. The binder holds the pigment particles distributed throughout the coating.The binder imparts properties such as gloss, durability, flexibility, and toughness. In sum, The binder is the film-forming component of paint.
I think this phrase used in the context of mixing different ingredients such as Binder as a way to formulate a paint.

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