Yup! You got it. The rubbing alcohol 70% should do it. It'll take off any residue left from the butyl tape too. "SCRAPE" sounds like a very harsh word. I hope your gonna use an old plastic spatula or something plastic and not too sharp to "SCRAPE" with??? Also I always like to wipe the alcohol off with a paper towel very shortly after prepping with it, just a precaution for painted surfaces. But it won't damage paint because it usually evaporates quite quickly. I also like to wipe the alcohol off with paper towel because the alcohol does leave a small amount of residue too.
P.S. If your butyl tape is really hard like cement, I like to use a heat gun or hair dryer to soften it up a bit. Just be careful not to hold it in one spot very long, as you may bubble your paint or damage the surface.
Hope this is helpful? Good luck with your new lights...
