Thanks, no its not the hairspray method, but a technique i discovered by coincidence.
First i cover the entirely exhaust in a thick layer of water thinned rust colored pigment and let it dry.
Then i make a mixture of burnt umber oil paint with white spirit (about 20% oil paint and 80% white spirit), and carefully apply it with a small pointed brush.
The pigment will absorb the mixture and the result is an uneven `rust` surface.
Because this mixture is very easy damaged just by touching it, i let it dry for a night and cover everything with a thin layer of Matt clear enamel.
Its a very simple but effective way too simulate rust.
Karel