I have a 100 pound LP tank that is collecting dust.

I want to use this tank as a reservoir for compressed air.

I would like to remove the valve, and replace it with a
much lower restriction ball valve. I'm assuming the original valve
won't allow much air flow, and probably not enough for an
impact wrench

My grandfather says it is all but impossible to get those
valves out.

Are the valve threads sealed with something that has
to be heated to soften?

Is there some other secret to getting the original valve out?

Thanks,

Dave