I had this problem on a car. I went to a local panel beaters, a chap I happen to know, and he gave me some liquid decanted from a large container. I don't know what it was, may have been acetone based, what ever it was it worked a treat.
Interesting, had same problem a few weeks ago, but after only 3hours!! .
Had devils own job removing it, acetone probably worked best. However! just went over boat with pressure washer to remove grime after craning in etc and I noticed that all the final little bits of masking tape aroiund paint edges just instantly softened and went away really easily.
Blue from B and q worked perfectly on next job