good waterslide transfers do not need varnish, it is already on them.
Think about modern auto paint - it is water based - does it wash off - nope, same with waterbased household paint, and the transfers once dryed on wont wash off either.
If you can afford to sacrifice one, apply it onto a not needed painted surface, let it really dry out for a day or two then try to remove it, it should be as well attached to the metal as the paint, will need solvent or sandpaper to remove.
My supplier states the transfers I use are made by the same firm that made them for BSA.
But if you feel better with another coat of varnish - no harm done
All the best - Bill