One of the biggest downsides of the Sony Mirrorless cameras is their ability to attract dust. When you remove the lens, there is no physical shutter covering it over, unlike on DSLRs like the Canons where the mirror and shutter stand between the sensor and the environment.
It really is a constant battle – even when you shoot wide open you will see the dust on your sensor as large smudges in your images. It is pretty easy to remove sensor marks on photos, but on video it is HELL! If you stop down, you will see every bit of crap on your surfaces, not just the sensor but on the rear of the lens and even the front surface of the lens. The closer to the sensor the dust, the more you will see it.
You can use blowers, but all they do is blow it off the sensor and to the side of the camera body, it will then fall back on the sensor pretty quickly. You can try mini vacuums too, and they can work.
I made the video below to show you how I deal with dust and crap on my sensor and lenses. There are also some links to the products I use below. The gel pen I link to is specific to Sony sensors. For other cameras, use their generic one. Do not use the generic one on Sony sensors.
Whilst this is super easy to do, I take no responsibility if you damage your sensor…to do so following my advice would be difficult to do. I have never had my sensor professionally cleaned, you don’t need to. It is SO easy!