Thank you both
While on a winning streak I decided to upgrade to some Roundhouse boxcar kits to fine tune the techniques.
First the model was given a clean in hot soapy water and mounted on a stand to reduce handing of the model.
Next a few tiny dots of neat red and orange oil paint was applied directly to the model. These were then brushed out smoothly into a film/filter to give various to the stark yellow of the original model.
Once the first layer of oil paints had dried a pin wash of heavily diluted brown oil was added around the raised detail, before it fully dried this was buffed and faded out with a dry brush.
Finally a coat of Dullcote was applied and rust chipping added with a fine tip brush and acrylic paints. The trucks were treated to some weathering powders and sealed with Dullcote.
