This is an idea that entered my mind. The traditional way is applying some etch resist like toner or dry film, etching away the copper and then adding solder mask before populating the board with components.
Can the solder mask be used as etch resist instead? It feels like skipping an unnecessary step in the process. Why isn´t this more common? This way you won´t need the step of removing etch resist only to replace it with a slightly different compound.
Adding solder mask is an optional step. If you use an etch resist like toner or dry film, you have a functional PCB even without solder mask. The solder mask is the unnecessary step that can be skipped.
Using solder mask as an etch resist, you would have to scrape away the remaining mask in areas where you want exposed pads - you would either need another mask for this, or to manually scrape each pad, adding an unnecessary step.