I stopped using WD40 under the vise on my 1100 series 1 mill. My reason was that even after a week or so, removing the vise revealed a table stain matching the contact area between vise and table. The stain could not be removed with anything other than abrasive paper which I chose not to apply.
I believed this corrosion/staining was evidence of dissimilar-metal corrosion, the table being cast iron and the vise being cast steel. I don't think the coolant enabled the this "electrochemical process" (see Wikipedia) because I was using a soluble oil coolant initially and then changed to a synthetic coolant, both of which I could leave to dry on exposed surfaces for weeks without any corrosion (as I could with WD40). But I thought it might be the WD40 acting as the necessary electrolyte.
I then purchased CorrosionX oil (
Corrosion Technologies) and now use it between the vise, rotary table, or any other metal worker-holder (including aluminum). No further corrosion/staining has occurred no matter how long I leave the vise etc on the table.
It would be nice to conduct an experiment to test whether WD40 acts as an electrolyte with dissimilar metals, with the variables of with or without coolant, dissimilar metals, and types of coolants. But I don't have the time or materials to do so. I have stopped any corrosion of coolant or dissimilar metals so I am content with my solution.
For what it's worth, as I posted in a similar "rust" CNCZone thread some years ago, the curators of the Australian National Museum have banned WD40 for use in maintenance of metal exhibits because it does leave a residue that causes subsequent problems and discoloration.
Bevin