Does the spray need to be reapplied to a mould after each firing and if not how many firings can be done before having to reapply the spray?

It is best to brsh off the loose primer and any flakey bits with a soft paintbrush and then apply a light top up coat for the next firing. You will see if the primer has flaked off then it will need recoating but if it only needs a small top up you can get away with using less.