Rust from PSD filters

Use the clouds filter with a very dark brown and a lighter rust color.

In the channels [black and white] make a new channel and apply the clouds filter to it ... the difference clouds a few times ... then the sketch/note pad filter.

Copy that channel and paste into a new rgb layer with DOLOR BURN as the blending mode ... there you have it, rust.

What is the difference between clouds and difference clouds?

Clouds makes a soft cloud pattern. Hold down ALT to make the pattern more stark. You will distinctly see the usage of the background and foreground colors. You do not need any color on the layer as clouds works entirely with the background/foreground colors.

Difference clouds ... the first application is similar to clouds, but as you repeat the filter, the image becomes more vein like or marble like. Also, the effect of difference clouds entirely depends on the color of the active layer. You cannot use a transparent layer. It is as if you the layer it is applied to has a difference blending mode effect on whatever pixel values are originally on the layer.