To be honest, and please don’t take this the wrong way, but I’d say you’re wasting your time.
You can get exhaust paint but in my experience with motorcycle exhausts the finish doesn’t last. Especially in an environment like under the car.
The only finish I knew of that worked was a ceramic coating that a local powder coaters used.
The exhaust gases are about 900 degrees centigrade when they enter the downpipe so you can imagine the temperature the paint would have to cope with.
And the exhaust will usually rust from the inside out, hot acidic gases passing through and a litre of water produced from every litre of petrol burnt by the engine. It’s surpising they last as long as they do really.
Hope this is of some help,
Martin.