You will probably get different opinions on this, but I would not use a rattle can primer for anything other than interior painting. With good, reasonably cheap epoxy primers and very cheap HVLP guns from HF, why not just do it right? The PPG Omni high build is probably cheaper than rattle can and while it is not the best, it will for sure beat any non epoxy primer from a can.
The one thing about rattle can primers is you can wipe them off with a bit of acetone or lacquer thinner so if you want to test a small part of the cowl, spray, check, then wipe off.