To eliminate the "paint ridge" of dope left from masking tape, I've tried scraping with a credit card and wet sanding with 2000 grit.  Both work, but if not careful when scraping you can do damage (and it may leave less than the desired sharp/clean line) and when sanding you might go completely through the paint, I've done both.  I just tried a new method my master building friend (Keith Palmer) suggested and it works quite well without any unwanted side effects.  After allowing the dope to dry for about 12 to 24 hours, wipe/gently rub the paint line parallel with a paper towel dampened with Naptha.   This removes the ridge and cleans up the jagged edge sometimes left when lifting the tape.  No guarantees here, but its working well for me!