That would cool the exhaust but also seems like a bodge. Ideally, the materials should accommodate the proper mixture, not the other way around.
I smoothed my manifolds using Roloc 3" sanding discs in 36 and 80 grits and coated them at home with CereKote. After a few thousand miles and 16 months they still look like they did when I first coated them. I bought the bare, uncoated manifolds from one of the usuals; SNG IIRC.