The beginner aeroplane modeler may be daunted by the challenge of adding exhaust stains, but for those wishing to create a truly authentic-looking model, it is necessary. Adding exhaust stains is part of "weathering," the process of painting a model to give it the appearance of actually having been used. Before you start, it is important to know how you want the model to look. Too much weathering can ruin the entire effect. A photo of an actual aircraft can help in deciding the colours, placement and patterns of exhaust stains.
Gloss-coat the entire model after the base paint job has been completed, and allow it to dry for 24 hours. This will prevent the next layer from eating through the base paint and the gloss coat.
Mix black paint with thinner in a ratio of about one drop of paint to about two droppers full of thinner. Test the mixture on a sheet of paper and adjust it to your liking.
Gradually apply the airbrush mixture where the exhaust streaks will be located. It will require multiple passes to achieve the darkness desired, but this process allows greater room for error and gives a more authentic look.
Allow the black base to dry. Make any additions or touch-ups to the paint job, then once again allow the paint to dry.
Clean the airbrush reservoir and make a new mixture of a grey or brown. Refer to the picture you are modelling your aircraft after and try to match the colour as best as possible. The colour with the next greatest surface area other than black should be the next colour mixed.
Gradually apply the next colour over the black exhaust-stain base. Allow the paint to dry, then make any modifications if necessary. Repeat steps 5 and 6 with the other colour of exhaust stain, if appropriate.
Make any final modifications, then seal the paint on the model with a thin coat of clear finish.
Make sure each layer is dry before applying the next.
Avoid over-applying exhaust stains and weathering.