How to control scale on plants
The sooty mould looks terrible but is not an active threat other than to reduce photosynthesis which will weaken the tree over time. It is the scale that needs to be controlled and the best way to do this is by using a white oil spray, such as Searles Pest Gun. Incidentally, check for ant activity as ants are usually the culprits for bringing the scale onto the citrus in the first place. The ants shift scale from plant to plant and they feed from the sugary secretions of the scale insect. So if there is ant activity in the tree, you may need to also control the ants or prevent them from accessing the tree.
One way to do this is by putting a grease band around the tree trunk which will prevent the ants gaining access onto the tree. Also check that branches are not touching the ground or other trees as this will also give the ants access.