Obtain a greyish, dark and matt aspect that you find on many engines that have had a lot of use. Apply with a brush or an airbrush to obtain different results.
https://www.robotines.com/en/outlet/2129-engine-grime.html2129Engine Grime<p>Ak-082</p>https://www.robotines.com/34916-home_default/engine-grime.jpg2.178instockAk-Interactive3.632.178401.4521341765575000/Paints and accesories/Paints and accesories/weathering/Outlet14786