Hmmmm.....
I dunno. When I get caught in the rain, which does happen (think decking a house when a big storm is coming) and cannot stop, our tools get soaked. Ditto when we go to a job and get caught in an unexpected frog strangler while sitting in traffic (no doubt caused by rain).
We use WD40 on the nails, not on the tools. Try this: spray WD40 on a piece of metal, and let it dry. It leaves a film behind. This film attracts dirt, lint, etc. and makes it into gum on moving surfaces. I sure wouldn't recommend using it on the tools, inside or out. My tool repair guy can tell in a minute when the armatures or brushes are gummed up that someone has been using WD40. That is why my favorite gunsmith told me to quit using it on my shotguns.
Acetone, etc., will dissolve or etch most plastics. I ruined some of my drill and saw cases as I was industriously cleaning off tar from the cases and I noticed the plastic became gummy. Can you imagine what would happen to the varnish on your motor windings if you did this?
As far as your nail gun goes, forget it. No harm, no foul. How much water do you drain out of your compressor tank every day? We are usually in for a few ounces, depending on the season. The point is this: is you are using a compressor to power your nailers and sanders and do not have a drying system of filters in place, they are already exposed to a lot of moisture. If you didn't have a funnel to channel the water into the housing, you should be fine. Many mornings in the south where we have extreme temp changes and high humidity you can actually see the condensation coming out of the exhaust port of the gun on the high usage guns like roofing nailers and staplers.
So what to do when they get wet? We dry everything off as quickly and completely as possible. If we have the compressor on site, we blow out the tools till dry, have some coffee and get back to work. Ideally, we dry with a cloth, blow them out, and then let them sit overnight before using.
Never lost one tool to rain as you described... lost plenty to rust, though.
Robert