i used por-15 on the underside of my camaro. used all the prep materials as instructed. te surface i actually sanded somewhat smooth, but from teh wire wheel it was rough enough. i went to the por-15 headquarters near morristown nj and spoke witht hem and told them i was gong to adhear it to a somewhat smooth surface.. and they showed me some samples that they did on smooth surfaces. what im saying is that as long as you use the chemicals as instructed (there is liek 2 pages of directions on prep work and whatnt)..you should be fine. see pics of my work here if you want to. i have no had even one hint of it peeling off even near the exhaust and exhaust cutout.
http://iroczman15.webs.com/2010carwork.htm