This is my own process, there are many...
Wash all resin parts and clean flashing. Use a warm dish washing soap.
Use testors 260 dull cote spray can for priming all parts.
Use badger water base paints and brush paint parts.
Use testors 249 flat black spray paint into a lid and them use a brush to apply to small parts.
Use krylon ruddy brown primer on frame before deck boards are added. This is the color used on the square water tank car and any of the on30 ready to run built ups.
Use various caulk powders applied by soft brush and then dull coted over.
Use about a 50/50 mix of early American Minn wax stain and thinner for wood items.
First weather wood items before staining by roughing up with an emery board. Beauty salons sell them for 99cents. Then dimpling for nail holes and stain.
Use a scribe for dimpling boards for small nail and screw indents.
Now apply the 50/50 mix or use a-west weather it for the grey weathered look.
Use Micro Kristal Klear for the window glass.
Drill holes inside window frames for slipping a brass wire into which will make the window pane frame.
16-24 up armor yellow paint by Badger is what I used on the climax a loco.
Krylon ruddy brown primer on the square water tank car.
Testers 249 flat black spray paint on the round water tank car.
Use the Krylon ruddy brown on all frames or your choice to make your cars various colors will make you train colorful and stand out.