To create this rust texture, lets start by creating a document of whatever size you need. Select the colors whoe un the pictture and then click on Filter-Render-Clouds.

The next step is to click on Filter-Texture-Grain and we will use the settings in the picture. You can change them if you want.

The next step is to go to Filter-Sharpen -Unsharp Mask and copy the data from my picture

Apply a dry brush with the seeting you see here Filter-Artistic-Dry Brush.

We now add a little noise to give it more texture Filter-Noise-Add Noise.

And for the last step, we will go to Filter-Artistic-Rough Pastels.

Here is the final result.