The 'historical' artifacts we love to build are often made from anything but the original materials. We use Precision Board High Density Urethane and fiberglass-reinforced concrete for much of what we fashion. This means we have to get creative as we age and even rust the final project. Even though our projects are very much a fun cartoon style they have to look believable when we are done. Thankfully we have a paint that actually rusts. The key is to apply it judiciously to our regular paint using brushes and sponges. The 'metal' surfaces have to look like they are many, many decades old and well used when we are done. After we apply the iron paint we spray on the rust reactive liquid and then wait for the real rust to form. The result is magic. It is real rust but it will never rust away. Amazingly all the paint we use is water based acrylics - no dangerous chemicals here.
These rusty relics will last for many years of hard service yet.