I used the Krylon high temp as well and followed the instructions exactly.
Removed heat shield, wash with light dish soap and warm water, dry, hit with light degreaser.
I didnt sand it. I just put several light coats on both sides. It only takes about 10 minutes to dry in between each coat. After several coats, I followed the instructions on baking the heat shield (yes, as in literally putting it in the oven) for the time specified on the bottle. If you go this route, open windows and doors and turn on fans unless you want to lose several brain cells.
OK...just an update. I sprayed mine with flat black fireplace paint. I cleaned the surface, but felt that sanding was unnecessary. I did two (maybe 3) coats, waiting 20 minutes or so between each and the whole thing took me an hour, including the removal, reinstall and waiting between coats.
Last month my son dropped the bike in the driveway and scraped off some of the paint. I re-sprayed the whole thing once and it looks fine.
The paint was cheap (actually free since it was leftover) and the process was simple. I definitely would not waste time sanding the surface, but cleaning it is a good idea. Assuming that the flat black is the texture you want, then what I did has turned out to be a simple and effective method.