Try some:
Gloss polyurethane like what can be found at a paint store or hardware store.
Liquid acrylic high gloss floor wax might work.
Do a test with the shoe varnish you mentioned to see if it works. Paint a small piece of the material you are using for the helmet with the paint you are using, treat it with the shoe varnish, and see if it gives the effect you are looking for.
