Thank you Alana for suggesting to use a roller when painting the shelf parts. The painting has been fast and smooth. But what hasn't been fast is the drying. The finishing paint feels dry after couple hours but the jar says that the paint is fully hardened after three days and before that nothing should press the paint. So I'm waiting and I've never been good at that.
As you know I still have the other shelf from the other wall to disassemble and paint, but I also bought more of that same shelf. Like I didn't have enough to paint before!