I have read on the net that wd40 is hygroscopic but I've also read that it's hydrophobic! Either way it seems it evaporates so is not a suitable long term protection coating. I am doing a test. I've 3 pieces of metal in my garden. One is naked. One is coated with wd40 and one with ordinary oil. Actually I'm going to add one with vegetable oil from the kitchen. I shall leave them all winter and see what happens. I'll report next year!
Nick