I Herculined the entire floor of my XJ, from the hatch opening to just under the dash and about 2 inches up the sides. There's no need to prime first, but you do need to read the label directions carefully and follow them. All surfaces to be coated need to be scuffed real good so as to remove all the glossy finish.
I had repaired some huge cancerous lesions over the cat and muffler and had some fresh steel to deal with, so I chose to coat that part of it first with some POR-15 to prevent another rust-out. Let me tell you that scuffing up the finish on that POR-15 was TOUGH. That stuff is amazing.
Use gauntlet-style chemical resistant gloves, safety glasses, long sleeves, and something over your hair. Any Herculiner that gets on you must be removed while wet with Xylene (and there's a reason you don't find Xylene on the ingredient list of finer skincare products) or if it dries, allowed to wear off in a couple of weeks during which EVERYONE wants to know what the hell happened to your hands.
It's held up really well - I love it.
Good luck.