It's not good long term for a tank to be under pressure all the time but the chances of anything happening are very very slim. I always purge my tank when I'm finished, after all it only takes a couple of minutes to refill. I always give tip it to and fro a couple of times while it's purging to get the water out, it's surprising how much comes oout after even one fill.
I leave mine pressured up, it has a pressure release valve as do most compressors so why worry?
As long as you do a monthly drain and check it should be fine, you don't see industrial compressors drained each day/week or even month.
You should purge the line/tank when you have finished as the air that is compressed heats up and then forms condensation in the reservoir and that in turn cause corrosion and water contamination in the paint. Big industrial compressors don't need to be purged as they are equipped with auto blown down valves.
I always empty the tank after I have finished painting, I also leave the drain valve open, I have been told that by leaving the drain valve open it stops condensation forming in the tank as the air temperature changes, where as if it is sealed condensation can build and cause rust to form. The industrial compressors I have been around had auto drains, that would purge the system every day to every few hours. We drain the truck air tanks daily as well. Just my thoughts
If only I could build kits at the pace I buy kits!
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I didn’t even know my compressor had a water drain until reading threads like this on the net. As a result, despite being over 15 years old, it was only drained for the first time recently...result, about 10 drops of rusty water.
My setup has a water trap/purge valve that I drain after every use, and I’ve never had any water out of that either (that I’ve noticed)
I bought my compressor from Richard at Modelair in the early 90s, and it’s similar to the “Platinum” ones now on Trade Me For about $300.