Well based on my past experiance with under coatings is that they don't really cover that well (there not paint) and if not painted first you will get little rust spots popping up. My plan is to prime, paint, truck bed liner, then under coating / sound deadener. The truck bed liners are much more durable then under coating alone , but don't offer any sound deadening. You'll hear every pebble on the road pinging of the bottom of the car. That's way I'll be going over it with the under coating.

I am just going to prime paint with rustoleum then use that 3m undercoating on it.   

My first piece of advice is to try to have any welding or anything else that will cause you to remove the bottom finish done first.  I wanted a painted finish, rather than an undercoating or lizard skin coating, so I looked for the toughest paint I could find.  I settled on Master Series Silver Primer - what they refere to as "permanent rust sealer" (a one part urethane w/ aluminum particles) topcoated with their AG 111 satin (a two part urethane).  You can see a pic of my underside in my recent post on adapting my 65's parking brake to MS's rear brake kit.  Here's a link to the Master Series products:  http://www.masterseriescoatings.com/index/

These are very tough, solvent-resistant, and UV-resistant coatings.  A benefit to using the AG-111 is that if you do have to grind it later, you can easily recover it w/ no special equipment, or if its just a small area  just paint over it w/ another paint of the same color.
 

I undercoated everything on my fastback.  Now I wish I had not.  A good quality paint is probably better and alot easier to clean. Unlike bedliner, I can at least get this stuff off if I ever need to.  There will always be ONE MORE THING you need to work on and that bedlinder is going to make it tough.  But, what the heck, I suppose it can be ground off if needed.

Another thing I have learned, flat black under a car makes it very difficult to see anything when you are working on the car.  My 69 coupe had red primer underneath and you could actually see what you were doing.  Like NASCAR cars, I think a grey or silver color is best between the frame rails where it doesn't show to the world, but makes visibility alot better.  When installing my door glass in the 66, I sprayed the interior of the doors with chrome paint.  Man it makes a huge difference in being able to see what you are doing!  The work light reflects all through the door.

I agree that a light color would be better.  I used black rustoleum and it is hard to see.  Whatever color you choose, you can mix in some microspheres ....

here's an oldie but goodie discussion on the topic.

http://www.hotrodders.com/forum/alternative-lizard-skin-103610.html