Most high-quality palette, or painting, knives are designed for flexibility, but the steel used does rust if exposed to water. This is not a problem while painting, but rust will occur during cleanup. If using water, or a damp cloth, to clean your knives, be sure to remove all moisture with a dry paper towel before putting them away. Use a cloth dipped in a bit of linseed oil and give the knives a light wipe to prohibit oxidation.
–Sean Dye, author of Painting with Water-Soluble Oils