You could also use invisible thread just in the bobbin. That would eliminate the polka dots. If you use invisible thread, it's important to purchase a good brand. I have had no problems at all using YLI monofilament nylon, which has about the diameter of a hair (.003 mm). Some brands of nylon thread are so thick they are more like fishline. A lot of people have switched to invisible polyester thread instead of nylon, and that is easier to find in local shops.