I tried using masking tape and a brush, but the paint crept up the tape edges, so the line was concave in cross section, where a brush/wheel leave a convex section. Didn't like that, so have only tried this once, for (in the pic below) the stay ends near the dropout and the thin blue lines on the downtube panel:
Since then it's either been decals or auto-pinstripe tape. Below isn't box-lining but it does show how thin the tape can be - it's the gold bands at the paint/chrome junction:
