Hi Michel -
You're going to be disappointed when you find out how easy it was...not much of a technique really. First, spray the exhaust with Model Master Buffable Exhaust. I think I may have even mixed in a smidge of Buffable Burnt Iron as well. Let it dry for about 5 minutes...it doesn't need any more time than that...then buff it with a Q-tip until it's smooth. Next...very important here...spray it with a thinned (60 thinner to 40 pigment) coat of clear flat...I use Testors again for that. Now let it dry for about an hour...maybe less. The flat coat gives the rusty pigment something to "bite" into. So...here's the big secret ...get yourself some Rustall pigments. Probably can find them on line through squadron or greatmodels. It's basically 4 numbered bottles. One is a watery black wash...I never use it. Two of the bottles contain a fine powder or grit...never use them either! The only bottle I use is #4...a rust tinted, alcohol based solution. I slather that all over the exhausts and let them airdry. That alcohol dries fast...leaving that oxidized look. One more coat of Clear flat and I'm done!