Those 3M Roloc discs are great on steel or cast iron surfaces but can ruin an aluminum sealing surface in a heartbeat. On aluminum, you need to use the least coarse grade available, use very light pressure, and keep the disc moving on the surface. Don't hold it in one spot to remove a stubborn piece of gasket, or you'll end up with a surface that won't seal, even with a new gasket. I personally stopped using Roloc discs on aluminum surfaces and stick with Jasco Stripper and a scraper. Paint on a heavy coat, and let it sit for several hours before scraping. I've also had good luck with Easy Off oven cleaner. If you read the fine print on a can of "gasket remover", you'll find it's designed to remove gasket cements and adhesives. It doesn't actually do much to soften rock-hard old gasket material.
Stu