Of the videos in Eggy's search, IMHO, one of the best is Lee Varis':
https://youtu.be/HLWh68q_yUI
Varis gives a very good, easy-to-understand introduction to banding problems, and then suggests the use of PS's splatter filter to break up the banding.
The spatter filter is a more effective version of the standard advice to add a slight bit of digital noise to the sky to break up banding. If you happen to have a good film grain simulator, ie, one that allows you to change the size of the grain, that's even better than the splatter filter, but the best film grain simulators (eg, Imagenomics' "Real Grain") are commercial plugins and are not free.
One of the other important take-home messages in his video is his discussion (towards the end of the video) of how smoothing filters, instead of helping with banding problems, often can actually make banding worse.
HTH,
Tom M
It's well worth watching the entire 20 minute video.