From a gun forum thread
http://smith-wessonforum.com/s-w-revolvers-1980-present/130140-wd40-cleaning.html"Back in the 1970's when WD-40 was much younger than it is today, there was a rumor that it would cause nickle finishes to go cloudy. In those years it was widely used on guns and many of us who carried daily in humid conditions and wet environments also sprayed the insides our holsters frequently. When that rumor came out I checked my 2.5" nickel M19 and sure enough it slightly cloudy around the high points of the cylinder, so I quit using it. I cannot say there was a definitive connection."
It seems WD-40 back then also turned to varnish when left on metal and allowed to evaporate rather than being wiped off. The solvent formula has changed since then. It seems Hoppe's #9 is also bad to leave on nickel. .
Cogar, I have no idea if the original story on WD-40 eating nickel was true or not but I like to err on the safe side. WD-40 is a cutting lubricant that displaces water. It is not meant to be used as a lubricant or as a protective film. There is much discussion about it gumming firearm actions up, disarming primers, ruining gunstocks, etc. on various gun forums. Used as it was meant to be, it is a very good product. I have about 5 cans sitting in the garage and shed that I use from time to time.
Anything with ammonia in it should not be left on nickel plating. Many sites suggest using ammonia in its pure form or cut down and actually soaking nickel parts in it but for no more than 5-30 minutes and then polishing with something like Simichrome. I would be hesitant to soak anything with a nickel surface for 30 minutes. When I use ammonia (sudsy) as a paint stripper on small items, I keep a bucket of water handy to immerse the item in and stop the action of the ammonia. You can't put back on what comes off.
