It is true that the thumbnail is the perfect tool for many scraping or chipping chores, but some jobs are just too big. When confronted with a job like this on a 60' racing shell (and only an hour to pull it off) I found an excellent substitute. I took an old credit card and scraped a utility knife blade along one edge. This creates a sharper edge with a bit of a burr, like a cabinet scraper but softer. The edge is easily renewed, and can even be shaped to match the workpiece. Rigid vinyl sheet, normally used for signage is almost as good but a bit softer. I don't know exactly what kind of plastic is used for credit cards, but it really duplicates the effect of a thumbnail.
Mike Fennelly
Alameda, CA