i think you should spray it with what is referred to as "HOT PAINT". this is sometimes also used on engine manifolds. you just spray it onto the exhaust or whatever and then go for a ride so that the exhaust will get hot. the paint 'sticks' when it is heated. also make sure that the surface is roughed up first, and i would personally clean it with thinner and dry it before painting to make sure there is no oil or dirt.
alternatively, do as G says and get a s/s exhaust wink