NO.
WD-40 is OVERSOLD when it is presented as a lubricant/preservative for firearms and other equipment.
WD-40 is really a penetrating oil, IOW, a very light bodied oil in a solvent carrier.
Light bodied oils tend to get where they're not supposed to be (you can kill cartridges in a gun if it has been treated with WD-40).
Jim, I certainly don't know for certain, but I don't think you saw "mildew" per se, I think you saw "oil rot."
The oil in WD-40 isn't very stable. When it becomes gummy, it actually becomes slightly hygroscopic, and apparently slightly acidic. Water and acid. Guess what that will do to the surface of your gun?
I've seen what Jim describes on a number of guns, and they have the following in common:
1. Heavily coated with WD-40 in preparation for long-term storage.
2. Left in non-climate-controlled areas.
3. The WD-40 coating was not renewed regularly. This apparently is a big contributor to the problems with WD-40. If you regularly wipe down your guns, you're renewing the oil layer, removing the old WD-40, and preventing a lot of problems.
In general, though, I do not believe that WD-40 has a truly useful application for firearms. There are other, MUCH better products out there, that don't have these drawbacks.
I use a lot of WD-40, though. Door hinges, rusted bolts, removing lable stickum, cleaning digging tools, in conjunction with steel wool for removing rust, etc.
It's a great product, but not for guns or other precision instruments.
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Smith & Wesson is dead to me.
If you want a Smith & Wesson, buy USED!