Here is a simply silly idea that works...The cracks are stress cracks from slamming the door. Also the cracks form from rust that works it's way up from the holes punched to accept the window trim. As example on my passenger side door, there were two cracks that formed.  I had two Mitsubishi Montero doors that were rusted out on the bottom and had been replaced. They were sitting in the Dumpster awaiting the trash pick up. I was going to work in the CJ when I looked at the radius on the doorsill and thought that it looked familiar.   I pulled the Montero doors out of the Dumpster and looked at the sill. It had the same radius. So I thought... could I cut out the good sill and tack weld it onto the CJ door? Would it fit? Yes, it did fit, like a glove! It took about 3 hours per door, start to finish, cutting out the Montero door sill, to priming and finish coating the rebuilt CJ door,  here's how I did it:
door_1 Small.jpg (11294 bytes) Here is the upper door frame, and how you remove it, if you need to. door_3 Small.jpg (5335 bytes) Here's why one of the cracks appear.
Door_2 Small.jpg (9525 bytes) Here's where to find the welds for the upper Door Channel. Door_4 Small.jpg (5603 bytes) Here's why more of the cracks appear.

 

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