K390 is not a stainless steel. It will rust without proper care. There are a multitude of good protective lubricants out there, and as mentioned above, wax will do a good job, too. In the days before all of the high tech protective lubes, we used to wax our rifles before a hunt so that we need not fear rain or moisture ruining a beautiful blued finish. That said, my K390 gets used around food, so I use Vaseline, which is paraffin wax and mineral oil. It will even lube the pivot if you want. It stays on the blade unless you eat an apple with it, or wash it off. I have never had any issues and I live near the sea and jungle.