Rust effect paint can be good, I haven’t tried any myself but I’ve heard they’re good for making something look really rusted and worn 
Otherwise just a bit of brown wash or a dab of watered down brown/orange in the recesses can work well for more minor areas of rust/dirt.
You mention above that you were hoping to get the resin set. I can’t speak highly enough of the detail and quality for the resin miniatures, they’re the best I’ve ever seen, but they are a bit harder to work with if you’re not used to working with resin. Make sure you give them at least one wash and dry (warm water with a bit of dish soap for 10 minutes, then gentle scrubbing with an old tooth brush) and have something like a sharp hobby knife for cutting off any flash (thin wisps of resin that sometimes appear along the mouldlines). There should be other threads in this forum that give tips as well
As a shameless plug, if you’re wanting to see more pics of my t60 suits I’ve posted quite a few here ( Alaiteir's painted minis ), though I’ve had to take a hiatus from updating it for a while