How to safely remove propane valve that’s rusted (how I purged the tank) 

Homemade penetrating oil better than store bought, and much cheaper: 50/50 mix of acetone (nail polish remover) and cheap ATF from the dollar store. A couple drops and let it soak into the threads for an hour or two, once it penetrates the rust, it should come loose a heck of a lot easier. I store mine in the bottle the NPR came in so the acetone doesn't eat the container. Strong stuff. :)

I have one of these but the bigger 40lb size i wanna convert to a volume tank for my air compressor...this is how im doing it for sure 👍

Try putting some light heat on the female threads on the tank. The valve should spin right out.