The study of factors influencing the adhesion properties of polyurethane coatings to aluminum has been continued. The data indicate that the addition of additives to polyurethanes can affect their peel strengths. The addition of epoxy resins, chlorinated paraffins, or sulfonamide–formaldehyde resins results in increased peel strengths; the use of a mixture of fatty acids resulted in reduced values. The isocyanate structure can also play an important role; those producing more flexible films (such as m-xylylene di-isocyanate) result in higher peel strength values. Other factors found to be of importance included the concentration of aromatic and aliphatic groups, the type of solvent used in casting the films, the crystallinity of the polyether glycols and the presence or absence of certain substituents, such as fluorine, in the polyurethane chain. Tracer studies have indicated that the mechanism of adhesion failure involves the breaking of the polymer-polymer as well as polymer–substrate bonds.