Regular Iso is slow, Rapid Remover is a lot quicker. If it's really heavy or hardened adhesive residue on raw aluminum you can experiment with harsher stuff like denatured alcohol solvent or acetone, they're cheap, cut adhesive fast, and evaporate quick. Wet it, let soak for a minute and use a scraper followed by another cleaning. But yeah, if it's bad you gotta figure your time against just replacing the panel, if you charge per hour at shop rate for removal, it may be cheaper to replace, if you're not charging them, you're losing money.