I am NOT a body man or painter but yesterday I was at the guys house who is doing major surgery on my panel. With most of the metal work done (the body is stripped/blasted) he sprayed a coat of white expoxy primer on it. I've seen primers before but this light color "lite up" every pimple on the body. My understanding is expoxy is the way to go for primer and for keeping the metal safe and in good shape.
Most body men have their own "technique" so lets see what the pro's come in and say!
Under the fenders some folks use the Por15, some use the 3m spray cans and I'm sure there are others that may hold up. For me I plan to use the quart of por15 I have on the shelf...but the spray undercoating may help with noise control if you don't insulate other area's.
