The wagon's lift gate is plagued with the same issue as the tail gate. It has a similar stance when closed, and as such, water lays along the inner window frame at the inside bottom.
I normally use an ice pick test.... if it holds an ice pick, it will hold paint. If not, it would have rusted through within a year anyhow, so you just saved ruining a paint job one year later..
Looking at our sample, the lower bend is fairly flat, going to a tipped flange in the form of an arc, the inside has some compound curves going to the window opening flange...
I'll attempt to form it in one piece, flange to flange, for a easier job of replacing.. Removing the upper gate...
My tool of choice for removing spot welds is a 3" x 1/16 cutoff wheel. I also use the premium wheels made for stainless steel. It's worth the extra money when you don't have a brown cloud in the shop, like the el cheapo wheels leave, nothing but resin wearing away.
The upper layer of metal will start to turn blue when it gets thin and hot, letting you know you're getting close to the next layer. When you have a blue circle with shiny metal in the center, you are now in the second layer, grind around the blue a bit more and pry the layers apart..
Here's a video to show better detail on grinding out spot welds....
http://s5.photobucket.com/user/rmccartney/media/1955%20Chevy%20Wagon%20Restoration%20Album%203/RemovingSpotWelds.mp4.html
With all the spot welds removed, here's what we find inside...
Note the deep pits. The big problem with just welding a hole closed is you don't know where the next pit is lurking, ready to break through your new paint.
Funny, with all the rust and scale inside, then you find this area of ~60 year old bare steel nice and shiney..
The inside of the remaining lift gate will then get media blasted to check for more lurking issues.
Here's a better look at the shape we'll need...