I always start by using single-edge razor blades and find that I can quickly remove most of the paint on the straight tubes. It takes a while, but it's quieter and less messy than using power tools. The lugs are a bit more challenge. The remaining remnants of paint/primer is then removed with a wire brush, or sometimes chemicals. I try to avoid chemicals because I don't like having to clean up that slimy residue from the frame.

Good luck on your project.