has been said the most important part of any paint job, be it vehicle or home decorating, it is the preparation that will give you a good finish.
You will need various grit rates of paper and not just wet and dry. judging by the roughness of your tail piece I would suggest something like an aluminium oxide low clog or production paper (240 grit) to start with and where possible use a rubbing block to ensure a flat finish (a block of wood will do).
Etching primers are usually used on materials such as aluminium.
For work on plastics I usually start with a high build spray putty, allow 24 hours to harden, then flat back. Then use a grey spray high build primer allow to harden, most will go off within the hour, then flat back with 360 Wet and Dry. The secret here is to use a couple of drops of washing up liquid in the water and keep the work piece wet at all times so that you don't 'tear' the primer.
Rinse off with clean water dry off and then check for pin holes. If need be reprime and continue the procees until you have the finish you expect the colour coat to be like i.e. super smooth.
It is not always required to remove all the old paint as there are such products called 'sealer primers' available which whilst priming they seal off all old paints to prevent a reaction with the new paints.
As most paint systems are now Acrlic (water based) it would be best to use this system for your base (colour) coat.
All clear coatings should be used with the greatest of respect as they will effect you lungs, and an air fed spray mask is the least you should use for your own safety. It should be applied in a well ventilated room with a filtered extractor system. (I do mine in an old tent I have but use the air fed mask).
If you have to do the finish coat in your garage then cover (dust sheet) as much as you can because the stuff goes everywhere.