I learned about this tip ages ago when I first went down the rabbit hole of all the good things that vinegar can be used for.
This is a quick-ish and easy thing you can. All you need is vinegar, a plastic bag and a rubber band.
Fill the bag with enough vinegar to cover the spout, bring the bag over the spout and use the rubber band to hold it in place. Leave the bag in place for 1 hour.
After an hour, take the bag off, use a wet rag or sponge to scrub the faucet and remove any residue.
Do this every 3 months and you will have sparkly down spouts! Enjoy and if you try it let me know what you think!