A flattening agent is a type of additive used in high-gloss paints and varnishes to reduce the gloss. Depending on how much flattening agent you add, you can achieve different gloss, satin, or flat (matte) finish effects.
Interlux makes separate flattening agents for its one-part and two-part polyurethane topside paints and varnishes. For example, Interlux Flattening Agent for Two-Part Finishes is designed to knock down the gloss of their high-gloss Interlux Perfection topside paint and their high-gloss Perfection Plus clear wood varnish to a semi-gloss or flat finish.
Interlux Flattening Agents (for one-part or two-part finishes) are solvent based and can not be used with water-based polyurethanes because they are incompatible.