Oh it's true. If it doesn't remove stains completely they'll at least be lightened significantly. If the first application doesn't get it do another, three at most because then you're just wasting time. let each application dry over night to let the acid do it's work. Once the stock is oiled they can be very hard to detect.
No need to get all suited up it's a very mild acid. It won't eat your skin or clothes or anything crazy like that, just gloves if wish but eye protection is always a must while using tools and chemicals.