Operating surface temperature of Prius muffler?

After seeing lots of rust spots on the muffler and the pipe before it this spring (see pictures #88 ~ 91 in this post
http://priuschat.com/forums/gen-ii-...-visual-inspection-dummies-2.html#post1115382
for example), I bought some high heat primers and paints. Yesterday I finally had some time to do the painting. The instruction on the can says the paint needs heat to completely cure. So this morning I took a drive around the local roads. 11 miles and ~30 minutes later I came back home and parked the car. I touched the muffler and found it just lukewarm. After breakfast I took the car on the highway at 55 ~ 65 MPH. 17 miles and ~30 minutes later after parking in the garage I found the muffler was warm to touch, not hot at all. Outside air temperature was 70F.

Thus my question in the title. Does Prius muffler operates at low temperature (no need for high heat paint, regular paint is fine)? Or does it cool down quickly during the ~ 5 minutes travel from the highway exit to home?

Thanks!