I'm not sure what you mean with powder residues?
: I have heard other speak of a varnish left by WD 40, but in over 25 years of use, I have never seen it and think it's just something someone made up.
: I would use a proper antirust agent if I lived in a humid area. WD 40 is great for helping flushout the moisture that might remain trapped in the vent or channel in a nippley gun. After flushing with WD40, one can always use his favourite protectant.