WD-40 over time will gum up and become more sticky than being a lubricant.
The WD-40 probably displaced the water, but it probably also displaced any oil in it as well.
If it were me, I would have taken an old school route and taken the grips off the gun, maybe field strip let it soak in kerosene for a little bit. This applies to metal parts, not plastic. I believe the Mark 1 is a metal frame, the only plastic is the grips iirc. This will displace all the water that got into it, as well as clean any nooks and crannies that don't normally get cleaned. After the soak, blow the parts off with compressed air. When the kerosene evaporates off it leaves a light coat of oil that will protect from rust. Would oil the volt anyway and put it back together.