[DanEC]

For what you are trying to achieve you don’t need primer. Just wipe it down good with Prep Sol or lacquer thinner and spray away. I lightly sprayed the bare underbody of my 66 Vette 25 years when I had the body off the frame, with semi-flattened black lacquer to blend in a fiberglass repair and for easy of cleaning and it has held up fine. Another option is spray bed liner which is what I used on the underbody areas of my ERA. Correction - I sprayed the wheel well side of my panels with undercoating and rolled/brushed bed liner protection on the body underside.