Clean your sensor.
If your going to attempt long exposure photography, you must make sure your sensor is clean, otherwise it will show every little speck of dust or dirt.
The image above had so many sensor spots I simply gave up, I had to crop the sky as that area had a lot and even using Lightrooms spot removal didn’t really help matters. It could have been quite a nice photo too. I’m no expert but I think different apertures show up sensor spots differently, maybe somebody could enlighten me in comments. So if you are going to attempt long exposure photography, especially if the scene is quite minimal as it is in this photo, check your sensor and if needed give it a good clean, it’s better to do that than faffing about later on the computer.
Btw, the photo was taken on the Northumberland coast near Dunstanburgh castle.