I am currently converting a second EMD GP18 for my C&E Railroad. I had this engine sitting in the spare parts box, without a motor, for a long time now.
But luckily I got hold of a spare motor and could revive this GP.
As a base livery, I choose the light green and black under frame. After spray-painting the body, I applied clear gloss coat to the sections where the decals will be placed. After the decals set, I over sprayed the body with a dull coat.
I opted for a bit more rust compared to the sister GP #1290. But in a first step, I fill the grills, louvres and deepening with a black acrylic wash. Also, the inside of the exhaust stack is painted black.
After this, I started highlighting some areas on the roof section with a light gray powder in several layers. The light gray is brushed on with a small flat-brush.
The rust marks shown below are done using powdered pastel calks. For sharp color edges, I use water to solve the powders. I started off with three layers of natural sienna. Followed by a dark, dull red tone. I blend the colors by only dipping the brush point on the spot where I want to have the pigments, this gives an irregular layer where there is some random color saturation.
Next layer is earth brown, only where I want to have older rust show up.
As a final layer, I highlight the oldest rust spots with burnt umber.