Yes, you could just tape it, but you will probably get better adhesion if
you scrape off the paint down to the sheetrock paper on the adjoining wall.
Then, use your paper tape in the corners. (I hate those metal inside corner
beads.)

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In a bedroom, I need to build a new interior partition 90 degrees off
an existing wall. The existing wall is painted (Benj. Moore eggshell
latex) and the new construction will need to be taped at the corner
where they join together. Can I just tape this corner like it was all
unfinished drywall, or will I need to prep the existing painted
surface to affect a good bond and sound tape joint?