How do I get a perfect finish to hair clips painted with acrylic paint?
A transparent protective layer would be ideal. I propose acryllic based coatings because they bond well with the painting and are soft enough to not chip away when bumped into. They are also elastic enough to not indent permanently, unless you have a go at them with a sharp object.
Glossy acryllic varnish or Mod Podge are ideal.
Instead of slapping a single, thick coat on the hair clip, you should apply several thin coats and let each coat dry completely. 4 or more thin coats, applied evenly with a soft and wide brush give you a better result than 1 or 2 thick coats. To avoid enclosing dust an other particles in the varnish, put the object under a cover like a plastic box to dry.