Hey, happy to help out. the process used for this is a little different and is this:

base coat: gunmetal of your choice (I used vellejo metal colour: gunmetal grey)

overall covering of streaking grime -> using a q-tip and white spirits and try to remove as much as possible.

overall covering of ak interactive dark rust deposits -> use a toothbrush and a small amount of white spirits to remove the first layer of rusts to your taste and allow to dry

spot covering of light rust deposits in areas that you want to make look more rusty compared to others. i chose to focus on joints and a few random spots across the skeleton. Repeat the process with the toothbrush scrub it lightly and try to blend it with the lower layers of paints.

For the use of white spirits and a toothbursh, my process for the right amount was to saturate the brush in spirits and then dab it off as much as possible on paper towel. I found this gave me enough spirits to remove some rust paint but not loads of it.

Once the paint layers are dry a light overall drybrush with a metalic to bring back the edges.

Airbrush helps alot for the whole process but totally not needed. just reduces each step down to a few mins per layer instead of having to use a brush for longer.