OK you will do this with materials - create a plane between the camera and your object. Make a material with an Image texture - your image should essentially be covered with “null” pixels, then paint on the “flecks”, “dust”, etc. You can do this in Gimp, I use this approach to get “dirt” on my models. the null pixels are transparent to the camera so it sees through them. Then make your animation and render it - you will not see through the plane until you render the output.
Alternatively you can also experiment with the “Transparent” shader using a black/white image texture - black will be transparent, white will not, I use this approach to cut “holes” in my models. There are a variety of option to explore, but basically it comes down to using Image based materials.