Hi folks, I have a slightly irregularly-shaped board (it's not a polygon - it has some slight curves) and I would like to have a margin inside the board outline that solder mask doesn't cover.    I have DXF files for the outer (same as board outline) and inner (same shape but inset a couple of mm) outlines of the area I want unmasked. I've played around can see that I can draw polygons on the mask layer that aren't covered with mask, but I can't figure out how to not mask the area in between two nested shapes.  I have also simply reduced the outline of my solder mask to a few mm inside the board outline and that doesn't seem to work either, which I find perplexing. Here is a decent example of what I'm trying to do:   [https://electronics.stackexchange.com/questions/293387/purpose-of-bare-copper-perimiters-on-hf-pcbs](https://electronics.stackexchange.com/questions/293387/purpose-of-bare-copper-perimiters-on-hf-pcbs)<br> <br> Ideas?