I have a feeling the answer to this is "you can't", but I'll ask anyway because some of my users don't like this. Neither do I, actually, and I can't figure out why it would happen like this.
We have tables with a black border around the outside and a thin grey border separating the rows and columns. The grey row border line intersects and overlays the black table border, so it looks like a series of dashed lines (if you zoom in enough). Oddly, the column borders do not do the same thing: the table border at the top and bottom are uninterrupted. See graphics.
So why is this, and is there any way to fix it?
My results match yours. The workaround I came up was to:
5. Set custom shading in the new columns to black.
Well, at least it's not just me. My systems tend to get massively customized, which can lead to some odd results sometimes.
I dunno about you, but I don't think FrameMaker should be doing this. Table Borders should always lie on top of row or column borders.
My user is also complaining that the table border lines aren't meeting smoothly in the PDF. I think that's a display issue, though. They're fine for me in both Frame and PDF, although at certain zooms they do look a bit off.