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This was by far the most tedious, but perhaps most rewarding, part of my restoration to date.

Refinishing the underbody began by wirebrushing every square inch of the underbody and rear wheel wells.  I removed loose factory undercoating, rust, original paint, dirt and anything else that had clung to the underbody over the last 20 some years.  And believe me, there was a lot of stuff.

After wirebrushing the underbody, I then cleaned the entire thing with a product called Marine Clean, manufactured by POR-15.  After everything was clean and free of oil or dirt, I then treated all bare metal spots with another product from POR-15, Metal Ready.  The Metal Ready did an incredible job of converting surface rust and providing a smooth, paintable surface.  Fortunately, I didn't have any major rust repairs to make, so the Metal Ready and a little elbow grease on the wirebrush removed all the rust.

After prepping the bare metal, I then coated the entire underbody with POR-15's "paint over rust" paint.  I chose the semi-gloss black because I knew I would be covering the POR-15 with another material.  Two coats were applied and plenty of time allowed for them to cure.

The next step was to reseal all of the body seams.  I used a general purpose 3M body sealer purchased at a local automotive body supply store.  It was easy to apply and even easier to clean up the mistakes I made.

Next, it was time to put on the final protective coating.  I went back and
forth on what to do with this part of the job before finally deciding to coat the entire underbody with a truck bedliner.  My options were to spray on an undercoating in a can, refinish the underbody with an acrylic enamel, or go with a roll-on bedliner.  The acrylic enamel option certainly would have looked nice, but I have a feeling it would have lasted just about to the end of my block before it was riddled with chips from road debris.  And since I want this car to drive on weekends, that wasn't an option.  Spraying on an undercoating, like that made by 3M, was a good option, but I wasn't sure how long it would last or how effective it would be in preventing the recurrence of rust.

So I finally landed on the bedliner option.  No, it's not OEM, and I wouldn't win any concourse events with this option, but again, I want to be able to drive this thing, not just tote it around on a trailer.  After a little research at local automotive shops and on the net, I finally went with the Herculiner product.  It's durable, easy to apply and it comes in a complete kit with everything you need to do the job (except acetone for prepping the surface).  The bedliner simply rolled on in flat areas, and brushed on in areas with difficult corners or nooks and crannies where a roller wouldn't go.

The result, I feel, is an almost impervious underbody that should last another 20+ years.  Best of all, since the bedliner isn't exposed to sun under the car, it should wear even better than the manufacturer claims because there's no UV degredation of the product.  Cleaning the surface is as simple as taking a hose to it.

Anyone looking to resurface the underbody of their Z should look into the roll-on bedliners. 
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