You just need to give the alloy a good smack on the top and bottom with a wooden/rubber mallet mate. Failing that, use a plank of wood and a regular hammer.
While you're at the painting, use a rotary wire brush on the hib faces and use black enamel paint on the hub to prevent it from sticking!
Also, if you havent bought the paint yet, id HIGHLY recommend Enamel paint instead of hammerite. Enamel paint sets much quicker and has a very smooth finish which makes cleaning them an absolute sinch!
I only did this a few weeks ago myself 😉