Thanks guys
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Process is very straightforward. After hollowing out the outlet to make the 'walls' more to scale, I sprayed a basecoat of Modelmaster Lacquer exhaust. Once dry, this was buffed to a sheen so it represented real steel. I then misted over a light and very diluted coat of Mig medium rust wash. Before this starts to dry, i then apply pigment. Using an old brush, i pick up some rust pigment and stipple it onto the exhaust. The wash will absorb the pigment, but by careful application, you can acheive the effect as above. Soot pigment is applied to the outlet. Plenty here as fuel quality was really poor in those days. A final coat of pigment sealer fixes everything in place.
Pretty simple process.
Gary