Rust in inaccessible places was the big reason behind removing the body from the pan. Years ago I had noticed surface rust underneath the car and had also seen evidence that the front firewall was beginning to rust out as well. The rot became fully evident once the body had come off and in the years since that occurred I had seen it get worse. Finally in Spring 2011 I was ready to get it fixed. Initially I had planned to get all of the body and paint work done within a few months time but an expected broken pipe and resulting sewage leak under my house blew my budget and changed my plans around. The most I was able to do was get some of the holes patched up and the underside of the body done. I'll let the pictures tell the story:

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From the A-Pillar back the heater channels were solid, somewhat surprising given the age of the car and how prone they are to rot. From the A-Pillar forward however was a different story. There were small holes in the bottom of both channels along with small holes in both front fender wells. And as the picture at left and the next one down shows the corners of the front firewall had rusted out as well.

I ordered a replacement front firewall from Mid America Motorworks. Every aftermarket supplier I looked at carries the same replacement piece of Danish origin. It's not an exact replica but it's good enough. Given that this piece will be covered with carpet and never seen when the Bug is done I wasn't very concerned about how it looked. As it was the welder that did the work only ended up using the corners. He initially planned to cut the whole piece out but discovered that area is multiple layers of metal sandwiched together and it ended up taking him much longer than originally expected to replace it. There's one shortcoming to the replacement firewall that needs to be mentioned. Inside the corners are pieces with nuts that move slightly where the body bolts screw in. The welder had to fabricate small pieces to replace these. At this point it's unknown if they'll match up exactly with the pan. I'll hopefully find out sometime soon.
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There were also several other holes in various spots of the body such as the one at left in the passenger side rear crossmember. The welder opted to grind these down to remove the rust and fiberglass them over rather than welding in patches. It was cheaper and easier and appearance wise not something I was concerned about since it'll be covered up and never seen. I just wanted to make sure the rust wouldn't come back. I don't have a photo of it but I also ended replacing the rear apron as well. At some point the Bug must have gotten hit in the rear and the existing apron had been fixed with a huge amount of body filler. After consulting with the welder I opted to order a replacement.

Once the Bug came back from the welder I put her up on a pair of sawhorses I built so I could have access to the underside of the body. The photo at right shows the before shot. I used a combination of my angle grinder, Dremel rotary tool, sandpaper, and wire brushes to to clean off the grime and surface rust.
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For the underside of the Bug I switched to using Rust Bullet instead of POR15. It's a bit cheaper and from numerous reviews and forum posts I read online it's just as good. Applying it upside down made quite a mess and wearing goggles to protect your eyes from drips are an absolute must. (Along with a respirator to protect against the fumes.) I still ended up getting it in my hair and arms though and just like POR15 the stuff doesn't wash off.

For a top coat I used 3M rubberized undercoating. It comes in a spray can and is very easy to apply. The end result looks a lot better than it did originally.
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After finishing the front I started on the rear end of the body underneath the luggage tray. Just like with the front I cleaned off the grime, rust, and old undercoating prior to coating it as shown below. The rear ended up being a lot easier to do this on than the front. There's more space and I was able to sit on my roller stool versus having to sit and maneuver on the floor.

Here's the rear end with a coat of Rust Bullet. If I remember correctly I ended up using a full pint sized can of the stuff between the front and back.
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And lastly, here's the rear end coating with 3M's rubberized spray on undercoating. More so in the rear than the front I'm hoping using that will cut down on some of the engine noise inside the Bug.