Provided is a chromium-free rust inhibitive paint for metals which is obtained by mixing a flaky zinc powder into a water reducible binder solution containing a water reducible resin emulsion and a water soluble silane coupling agent. A thin film of a silane compound having a hydrophobic group is added to particle surfaces of the flaky zinc powder, whereby the flaky metallic zinc powder exhibits water repellent property. When a surface treatment agent which contains an alkoxysilane oligomer as an essential component is applied to the paint film of a rust inhibitive paint for metals, this extends the time until red rusting occurs on painted metal products to not less than twice and prevents white rusting and black rusting for a long time. Rust inhibitive performance is further improved by blending a nano-sized powder of titanium dioxide, polyvinyl butyral, a silane coupling agent, etc., with the surface treatment agent.