Pete's advice is probably best for small cracks like these, as the thin super glue will penetrate nicely into the cracks. For bigger cracks and voids you can make a mixture of epoxy resin and metal powder. This should be about 2/3 powder and 1/3 resin (which has to be pre-mixed first!) to give a thick paste, which can then be pressed into the cracks/voids.
There is a fine balance between getting a paste that will fill the voids, i.e. not too thick, and getting a decent metallic surface (which requires a high metal powder content). Pretty much any decent epoxy resin will do the trick, and since you are using a metal powder, the colour of the resin is not important.
There are also epoxy resins available that are clear and can be mixed with colours to achieve a different effect. Google is your friend, but be prepared to spend around £50 for a decent kit.