Sanding round shapes. A sheet of thin spring steel works really good, it flexes enough to make the curve, yet since its steel, it still cuts the filler good. Thinner acrylic (yellow handle) also work for some curves that aren't too tight. An advantage to these types of materials is they always make nice natural curves when they are bent, unlike foam or anything like that. **Note: a piece of sandpaper on the back side of the spring steel makes it a lot easier to hold on to. **Note Note: notice all the places on just the green suburban this would work great with.