Here's a before/after of the backsides of my rims. I had to resort to steel wool (and a brass brush in places) along with Orange Clean and Mean Green. I even gave the rims a shot of oven cleaner and it didn't even make a dent in the stuff.
Afterwards, I put a coat of NXT followed by a coat of #16 that was allowed to sit overnight. Here are my results:
Before:
After:
Afterwards, I put a coat of NXT followed by a coat of #16 that was allowed to sit overnight. Here are my results:
Before:

After:

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