Modeling Rusty Bridges

By Keith Wilhite

Although it is possible to create the appearance of rust by using paints, chalks and other weathering materials, I discovered another way to do so during a visit to a paint store a few years ago. Located among faux finish paint products made by Modern Masters, I discovered a product called Metal Effects Iron Paint and Rust Activator.

The Iron Paint is a dark grey waterbased acrylic, which suspends the microscopic particles of genuine iron. It is a rather thick paint and will need to be thinned by as much as fifty percent with water, in order to pass through an air-brush. Only external mix air brushes are recommended, due to the thickness of the paint, even when thinned. It will require a fair amount of air pressure to move the paint, from thirty to forty PSI.

The Rust Activator is an aqueous solution of copper salts, and accelerates the rusting process after the paint is dry. It is not harmful to plastic. I tested the paint and activator on a piece of Styrofoam insulation board before applying them to a model bridge, with no problems at all.

The paint and activator are a bit pricey, at about $10 per six-ounce jar of paint, and about $4 for the four ounce bottle of activator. Since the paint will need to be thinned by fifty percent to use, though, one jar could be a lifetime supply. It will probably dry out before you could use it all, so share it with a friend!

I use a Binks Wren Type A external-mix airbrush and find that it takes quite a while to paint a bridge. The paint just doesn’t move very quickly. It will get there, eventually, and the result speaks for itself, so keep at it!

The Iron Paint will be a dark grey until you spray on the Activator, and then it will take a while to start working. It is best to prevent large pools from collecting at the bottom surfaces and corners unless that is the effect you desire, as shown at left. By soaking up excess activator, the rusty finish will tend to be more even.

The texture of the finished rusty surface is quite authentic, because it is real rust! Check out the photo below:

It is not necessary to prime your plastic model, but doing so will not hurt anything, either. I prefer to prime with a reddish zinc chromate primer, available from Scalecoat Paint. It might be interesting to allow some of the primer to show through the Iron Paint as an additional detail, but I have not done any experiments to find out how it would look.

I hope your project to model a rusty bridge works out as well as mine did. The Iron Paint is a very interesting product with much potential for our hobby.

© 2009, 2019, Keith Wilhite,