My opinions:
Don't buy over a quart.
Don't open the lid.
Buy some cheep clear tubing at the hardware store, a little larger than your biggest Phillips screwdriver, punch 2holes in the lid, push ends of a 6" length of tubing loop to 'close' it. If you open the can, humidity in the air will jell it up overnight, even if only open a minute( in ohios muggy air anyway). Pull out one end of the loop, pour a little bit into a plastic flexible container, brush it on(harbor freight sells cheep chip brushes that work fine). When done, leave brush in the leftover paint-next day pull it out in a chunk, it don't stick to plastic. I had a can last 2 yrs by pouring this way, was the only can I actually was able to use all of before it jelled up
Remember, por don't stick to paint. As far as their prep, IMO 'Ospho' brand at ace hardware is just as good at pretreating rust... They're both phosphoric acid, all they do is convert iron oxide to iron phosphate, which stops rust pretty well on its own, but also gives incredible bite to any paint...por over it and you'll need a grinder to get it off.
Por looks like crap if the sun hits it...within months it's like charcoal, stops rust, but ugly...their "chassis black" bites onto the por, and it's a nice flow out semigloss, think I used 2 qts on the entire frame and underhood area, still looks good 8 yrs later, I'd guess it was closer to 3 qts of the por actually on the car, but I know I ruined at least that much to jelling before thinking about the tubing thing...
Last thing- it is the absolute messiest stuff you will likely ever touch. One drop on your shirt, will wick out over your entire chest and stay for at least weeks, it don't wash off. Drip it on concrete, it's there forever. Brush it over painted areas, it will curl/peel up in days...it's weird- not 'paint' but moisture cured urethane, with phosphoric acid or something in it to make it somewhat self etching... Neat stuff, but needs over coated to look good, and for goodness sake, wear goggles/rubber gloves if opening/piercing the can.
Every time, careful as I could be, I somehow still wound up with this crap all over my hands for weeks...
Start out with a small can- it's amazing how far it goes.
For some pics, these a bunch here, underhood stuff towards last pages I think...everything black is por/chassis black except the engine block/intake manifold:
http://s301.photobucket.com/albums/nn78/ford4v429/galaxie/