For questions about component-level electronic circuits, tools and equipment.
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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.
That is a good point. I was under the impression that solder mask is mostly for safety and convenience when soldering so it only flows where you want it to.
Creating a mask that works for solder mask as etch resist does require some tweaking with the SVG files. I'll give it a try for a few small boards to see how well it really works.
You'd still need to apply a second layer after the etching. There's less risk of misalignment if the solder mask is all applied at once, so might as well use the more appropriate resist.
Why is that? I understand there is a tiny layer of exposed copper on the side of the traces but copper tends to form a protective layer of oxide.
Here's a video where one guy already has done this successfully, even with diy silkscreen https://youtu.be/gSjTScsCXRM