I see lots of folks recommending blowing out the main drain with air, then using the drain line valve to lock in the pressure and keep the water out of the drain line. Is this only for inground pools where the drain lines are probably below the frost line anyways? Would this be suitable for AG pools too?

If not, I'm prepared to put some RV antifreeze in the MD line, but since it will be mixed with the water in the line, how much do I need? Do I then keep the valve open (or remove it) to allow freezing water in the vertical pipe to move, or close the valve for the winter.

My PB told me to use pool store anti-freeze, not RV anti-freeze, but I think he misunderstood my question, assuming RV anti-freeze is automotive anti-freeze.

Thanks!

ps: my main drian piping is PVC, not flex pipe.