What is everyone doing to stop slow tire leaks?

The rear tires, 26" Bighorns on Polaris Crusher rims, on my 2013 S leak down slowly. I can pump them up to 12 pounds and over a couple weeks they will leak down. I seen them go as low as a PSI but never completely flat. I had a tire store check them for puncture and they couldn’t find anything. I had them remount them with a good quality sealant and they continue to slowly leak. The fronts stay at 12 lbs with no issues.

I thought about using Slime in them but I wasn’t sure if it would damage the rims. Are the Polaris Crusher rims aluminum, steel or something else?