As for the weathering I used the brushes below and use Vallejo acrylic paints. The paint dish is one I inherited but an old saucer will work just as well. The kitchen towel is used for wiping brushes.
I have more brushes but used these four for the GP9 weathering.
Acrylics tend to dry out quickly so you can always create a wet palette by placing the paint on greaseproof paper on top of a damp kitchen cloth in a Tupperware container to keep the paint moist. My wet palette is one I purchased a few years ago.
My technique is a mixture of solid painting, washes (using water or Vallejo airbrush thinner), dry brushing and stippling/dabbing.
For the larger rusted areas I used the light rust and the large Daler-Rowney brush. I put some paint onto the brush and wiped this on the kitchen towel as if I were going to dry brush.
Instead of dry brushing I dabbed this onto the model which created the rust patch and created a feathered edge. Once dry (well, it dries almost immediately) I went over the centre of the rust patch to intensify the colour. In order to create some variation I randomly added a spot of German camouflage brown, dark rust and light rust into the mix. On SSW/SP diesels, as they were not cleaned in the period I'm modelling the rust around the cab and short hood is probably the primer after the grey has worn off. The paint was also applied over the numbers and I cleaned these afterwards with a cocktail stick.
Part of the battery cover here was painted with dark rust using one of the smaller brushes. I then loaded dark rust onto the Daler-Rowner brush, wiped it dry and then dabbed over the dark rust.
The area of the battery acid stain was painted black and then I created the colour gradient with washes. After this was dry I applied a thin dark blue wash.
On the driver's side the battery box cover was just dabbed with dark rust. Unfortunately the nature of the lighting I have and the tablet camera don't appear toshow this too well.
I tried to demonstrate this on a boxcar but the washes did not work too well. The boxcar was sprayed with Testors dullcoat but this did not take the paint too well (also looks a bit orange peeley in the photo). The GP9, on the other hand, was sprayed with Alclad matt varnish after the decals were applied and this seems to be a better surface on which to weather.
Using the smaller brushes the rust patches are a dot of German camo brown or dark rust. I then use light rust to add the streaks. These haven't worked here. The boxcar will eventually be stripped when I rebuild it to either a SOO Line or ATSF version.
This was a HO SOO Line insulated boxcar I converted and weathered many years ago which shows the rust streaks.
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