I've found that a wrap of WD-40-soaked paper towel, covered with foil to slow the evaporation, softens hard tape adhesives overnight. Best of all, it's safe to leave on paint for days, at least on all the bikes and motorcycles I've tried this on. Hardened Masking or "duct" tape residues are also no match for a 24hr soak in WD40.
As art mentions, heat will soften the adhesive in most cases to the point where it'll come off with a plastic scraper or fingernail. Also, as Tom Sanders mentioned, any solvent will need to be able to reach the adhesive behind any continuous plastic substrate(s).
David Snyder
Auburn, CA usa
Subject: Fwd: [CR]? How to remove 3M reflective tape and its adhesive ?
>I have successfully used a hairdryer on reflective tape put on the front
>forks of a 1940's bike. Art Link,San Antonio,TX,USA
>
> travis.harry wrote: > To: <classicrendezvous@bikelist.org>
>
> How to do this for tape that may have had over 30 years to bond to
> anodized aluminum? Asking here, as it is part of a restoration project.
>
> I've tried typical automotive cleaners and solvents on residues after
> scraping. But, it seems the folks at 3M made their tape and its adhesive
> resistant to them. Nor have they responded to my inquiries.