I can't understand how this would work, since there seem to be no rust-dissolving ingredients in WD-40 and it seems the product is more basic than acidic.
A (possibly not exhaustive) list of ingredients:
- Mineral oil, I guess for lubrication and rust protection layer (but not a rust solvent?)
- Decane, freeze-resisting
- Nonane, water-displacing
- Tetradecane, freeze-resisting and water-repelling
- Undecane, freeze-resisting and water-repelling
- Dimethyl Naphthalene (C12H12), likely used as a solvent
- Cyclohexane, solvent
- Carbon dioxide, likely used as a propellant
I can imagine that the low viscosity of WD-40 allows it to get in all nooks, making it easier to grind rust particles away because it acts as a short-term lubricant. Is that actually how it works, or am I missing something?