The properties of steel substrates coated with soft polymers were characterized, in order to assess their connection to ballistic properties. An impact-induced viscoelastic phase change of the polymer effects large energy dissipation, while also spreading the force over a wider area, which reduces the impact pressure. Both effects enhance the performance, as directly measured and seen from strain measurements on the substrate taken during ballistic tests. The contribution of the front-surface polymer to impact performance is increased for harder substrates, indicating a coupling of the layers related to impedance mismatching. Since this effect is very local, the phenomenon can be exploited by surface-hardening of the steel.