Designer Ken Wingard and I worked together recently on a retro marquee sign wall prop DIY project that although was actually made of wood, had a vintage metal faux finish treatment that sold it as the real deal. It's a super simple trick I picked up from a scenic artist I worked with many years ago.
Step 1 - Base coat your prop in a dark brown or similar dark rusted aged metal color paint. Go thick with. Once brushed on, pounce it, creating a more textured surface. Let dry completely.
Step 2 - Pour ordinary grocery store salt over the areas on your prop you want to look aged (for this project we used heavier coarse ground salt, but finer ground would work too). For vertical portions, you can defy gravity and make a paste of salt and water and brush on.
Step 3 - Spray paint your prop the color of choice. Let dry.
Step 4 - Brush away salt. You can leave rough edges of paint infused salt or refine the faux finish with some sand paper. Any overall dusty haze the salt leaves easily wiped away with a damp rag or sponge.
There ya go. An easy way to give your props a fairly realistic aged painted metal look.