No, we don't recommend it. Water-based mediums interact with the water-based varnish in undesirable ways. Users can expect smearing or, more commonly, ink bleed where it seeps.
Instead, we recommend using a specific archival varnish for watercolor instead. A coat of varnish can be applied in a similar manner.
Remember: Timeless and G2 are formulated specifically to work with IRC [inkjet receptive coating] on our media.
PRO TIP: When in doubt, ABT --"Always be testing!"
While we recommend using Timeless with inkjet media, we also encourage creative experimentation. We recommend applying Timeless to a couple of test prints first. This way, you don’t run the risk of damaging an original.