I've heard good things about POR15.

But it all depends where the rust is too.

 

 

Ash are you talking about the exhaust rust?
If so you might consider getting the system ceramic coated at your local powder coater ,
or you can try Jet Hot coatings.
Or the easy way buy a few cans of hi temp spray paint and recoat the system then drive the car to cure it

 

 

they are off a bit but it looks like a hand made system