Oui, Monsieur
When air was compressed in the tank, there was a water condensation, and water is still here. So what is important at the end of the painting session is to empty the tank as to eject this water. Even if the tank was painted inside ( you put a quantity of rust protection paint, close the opening and shake/roll the tank to cover all areas ), it would be much safer to empty it. I don't know if the fact of having it under pressure would damage the tank and the gauges in the long time.
P.S. If you have a "tap" under the tank, use it to empty it as water is on the bottom. I also noticed that my pressure gauge on the regulator went to zero when I released air from the airhose outlet, so I open both at the bottom and at the top of the compressor.