Aluminum is available in a very wide range of alloys, just like steel. The properties vary just as much too. If you want real scratch resistance then for parts like that use 7075-T6 alloy. It is harder than mild steel and stronger too. It also costs more. Another alloy with similar properties is 2024-T4. Also as hard and as strong as steel. To resist weathering it does need protection of some sort.One of the other problem I find is how soft a turned part is. It takes one fumbled drop or clank and the surface gets nicked. Is all aluminum this way and I just haven't noticed, or is there a typical hardening process 'professional' parts typically get?
I will warn you that if you don't know what alloy you are using you run the risk of using something that is not strong enough for the job. Pure aluminum takes a very high finish when polished. The only element with higher reflectivity is silver. Unfortunately, pure aluminum has only about 1/10 the strength of mild steel and if used for something like those standoffs could be easily bent.
I go along with the objection to clear coat. Once it becomes cracked or scratched it looks like garbage. Instead use regular car wax and polish as required.
Aluminum alloys come in two main varieties, those that can be hardened by heat treatment and those that can be hardened only by cold working. As for "professional" parts, many are made from regular 6061-T561 alloy and then anodized. Anodizing is a very thin coating of transparent aluminum oxide that may or may not be dyed different colours. It is a very hard surface and is very resistant to scratches. I use it to protect parts that will see heavy use. Example: