Depending on the coating you might not be able to powdercoat it anyway.
Martin Brown Paints do small amounts of RAL mixed paint you could apply with a foam roller for a very good finish - like coach enamel. Think it’s the “brushing synthetic” I’ve used.
I actually found the perfect off the shelf product: ‘Tamlyn XT-VERT profile’.
Can be had from here, among other places: https://www.guttersupplies.co.uk/aluminium-vertical-trim-3mtr-black-p-XT-VERT-MB/
The only issue now is the lead time to getting it powdercoated in a custom RAL…