Stoddards sells the factory grey VHT exhaust paint.
If you have your exhaust sand (or bead, I'm not sure what they use) blasted so that its is totally clean, and apply the Stoddard paint with a paint gun per the instructions, you will get a lot of life out of the paint.
It is the only method that has worked for me over the years.
I did my 914 that way, and the exhaust still looked great after 50,000 miles.
It REALLY works. But you MUST do it correctly. If the exhaust is not blasted down all the way to clean metal, it won't work as well. If you do not apply the paint with an automotive paint gun, it'll go on too thin and won't last as well.
Here's some pics of my former SC, done the same way.