To clear-coat these roof rails, or not?

Seattle-ish, WA
I refinished my roof rails today (or partly so). They are cast aluminum and have a rough finish, had black paint on them. I've applied about three light coats of Rustoleum paint and they have a nice finish, but not deep. I did the cross-wise load bars, which are the same aluminum.

Should I add a little more paint? Clear coat? Leave as-is?

Full disclosure, the paint says on the back, "clear coating not recommended." I have no idea why - but it does NOT say, "do not clear coat." The thing is, they make a clear coat in this they recommend over color - so it doesn't make sense. Only thing I can find wrt that is this conversation:

Where some guitar restorers talk and one says the issue is people get the clear too thick and lift the color coat (?).

I just want it to go on the car and resist chipping, and resist staining and fading.
I wouldn't, after the paint dries you run the risk of the new paint lifting the dry one like paint remover, you can re-coat within an hour but not much longer. This is true with most rattle cans and especially Rustoleum products. After 30 days it is possible.
Lacquers, urethane and 2K stuff is different.
Caldwell Idaho
Whywould you want to clear coat the parts? I have used clear coats on paint that required clear coating. Does the paint require clear coat?
If you don't need a gloss finish, you CAN clear coat by spraying a lot of light layers (never put down enough to produce a "wet" layer). It'll take many applications to build up, but you won't risk harming the base coat. The finish will have a lot of orange peel but you'll protect the color.
DFW Metroplex
What kind of paint was the Rustoleum color? Lacquer, urethane, epoxy, etc?

Using a lacquer clear coat over a non-lacquer color coat will cause the failure of the color coat like mentioned, and using a non-lacquer clear over a lacquer color coat could reduce the clear’s longevity, as it may not adhere very well and delaminate after some time.

If you do clear it, make sure to use the same paint chemistry clear coat as the color coat, otherwise you’re working against yourself.
What can specifically did you use it would be the only way to answer the question. Without that answer you potentially risk recoat lifting.