The geometric stiffness of structural sheet metal panels, such as those employed in motor vehicles, may be increased, without major increase in the mass of the panel, by application of a coating of a structural polymer. However conventional means of applying such a polymer layer require multiple applications to achieve a desired thickness. By incorporating magnetic particles in a resin, and subjecting the resin to a magnetic field as it is applied, the desired thickness may be achieved in a single application. Embodiments appropriate to the application of such a resin to both magnetic and non-magnetic materials are described.