When you finish the claybar.....What if I applied Rain-X to the entire rear of the car (not just the lower rear window) -- Any reason not to try that? Might that not help to repel some of the dirt?
Start with this: Wolfgang Polish Enhancer
And finish with this: Wolfgang WG-5500 Deep Gloss Paint Sealant
I was amazed at how much less dirt collects on the rear, and overall. I've gone through rain events and not needed a wash afterwards. Makes the paint super slippery.