On Mon, 1 May 2017 15:50:09 -0700 (PDT), "
wrote:

On Monday, May 1, 2017 at 5:51:19 PM UTC-4, wrote:
WD-40 does not completely evaporate over time unless the temperature
is higher than that which is compatible with human life.

No, it does completely evaporate. That 'leftover' is the previous skunge dissolved and spread over everything.

To prove this out:

Take a standard paper towel, clean and dry.
Saturate it in WD-40.
Leave it on a clean surface in a location in your domicile exposed to normal temperature variations.
Check it in 4-20 days depending on ambient temperatures.

Don't take my word for it.

Peter Wieck
Melrose Park, PA
Peter, I discovered this residue after cleaning a machine well with
solvent and then spraying wirth WD-40. There was left behind a hard
coating on all the previously cleaned metal surfaces. I have since
learned to use this to advantage when repairing an oil rubbed bronze
finish that has been damamged by machining operations. After texturing
and oxidizing the bright bronze machined surface I heat, then spray,
then heat and then spray again with WD-40. This ends up being almost
exactly a US-10 oil rubbed bronze finish and wears identically.
Eric