The blades are very hard...
...that means they're also brittle.
Rounding the edges of the blades to avoid scratching the gelcoat only worked with the grinding stone I normally use to sharpen my drill bits. The Dremel with a carbide burr failed miserably.
The problem with the hardness or brittleness became even more apparent later when scraping. I gradually got more and more "gaps" in the scraping result. Eventually, it got to the point where I had to rotate the blade because the result looked like I'd used a comb. At some point, I realized that you have to place the blade gently on the gelcoat; then it lasts longer. As soon as you apply too much pressure, pieces break off, and you get the corresponding comb-like pattern.
The scraper itself isn't exactly cheap, but it's the best tool I've used so far.
I removed the first few layers because of... I scraped the hull curvature lengthwise down to the first antifouling and primer layer applied by the shipyard using a vacuum. I removed the rest with antifouling paint stripper and a scraper. Since the antifouling hardens again with the stripper, I had to clean the scraper frequently. However, since it was easy to disassemble, this wasn't a problem. Finally, I cleaned the remaining primer on the hull with 180-grit sandpaper, and that was it. The scraper and blades are great; you just need to know, or learn, how to use them.