Primer for bare metal on my 67

One thing you can do for your floor pans is scuff them before you put them in, that way you can do it easy, they are usually coated in black primer and for good adhesion all you need is a red scotch bright pad. Get them to fit, punch or drill your plug welds then scuff them up. What do you plan on doing for the bottom of the car? Painting it? If it's not something you are painting body color I wouldn't waste the cost of epoxy or sandable primers on the floor pans. I have always used the NCP primer over bare metal with no problems, it is very high fill, does tend to clog paper up a bit when you sand it because of it's clay content. It's a pretty thick primer so it works well for board sanding before paint. You can put a little reducer in the first coat to let it soak into filler and sand scratches. I don't like to suggest etch primers as they are really terrible for your health regarless of resperators worn.