John,
you might like to try a “broad brush” approach.
Once the formation is painted grubby timber colour, put a length of masking tape over the timber ends, and have the airbrush both close and very low, to paint the chairs and rails with your preferred rusty mix. Allow the rails themselves to mask the timbers within the formation. Any overspray can be touched up by brush. It’s certainly not precision, but it gives an acceptable result for a relatively limited effort, in my view.
I recall a post on the Templot forum about using different colour filaments in the print, the timber layers in brown and the chair & rail layers in presumably orange. I think it was in relation to CARROT, but presumably applicable to COT, neither of which I have yet tried.
I’m still in the “learning Bambu” and “relearning Templot” classes
best
Simon