Just to share a different experience, I've been using Vallejo for several years now and have never used a primer and have not had any issues with adhesion (outside of blatant abuse  ), including 1/35 resin figures.

Where practical I wet-sand with 600, then wash everything (dish soap).  Seems to give the paint all the bite it needs.  Above all: give it time to cure!  Its dry to the touch in minutes (sometimes seconds) but a typical coat needs at least 24 hour to fully cure (even more if you do an extra heavy coat or put on several layers in one session).  During that cure time it will indeed be easy to damage so you just have to be carefull or patient (or both).