I use a gasket scraper which I bought many years ago. It's a cheap thing but I spent some time flattening the back and sharpening the bevel on an oilstone. If you use one of these, bevel down, with the bevel parallel to the surface, it cannot dig into the aluminum. For softening, I use lacquer thinner. Gotta say, though, I've run into some gaskets that were an absolute buggar to get off. I would never use an abrasive on a gasket surface.