What I use on exhausts, believe it or not, is White Night Hi Temp from Bunnings! There's a few colours, and I have never experienced it smoking or discolouring, a slight smell for a little while though. It's the best I've found. It's what I used on the Mobilco, and that pic was taken after I'd run it for about 2 hours! It's a metallic charcoal colour.
As far as prep goes, all I did was hit the muffler with an angle grinder fitted with a wire cup wheel. You can see in the pics what the muffler looked like before.
Worked a treat, and it stays there too. I've tried a few different brands, nothing seems to come close.
