Steel frame? Sandpaper will work, but it's a tedious job, and hard to reach tight spots. Chemicals are messy, toxic, and also tedious. Quicker to take the frame to someone who has a sandblaster and remove the paint that way. Once the paint is off, consider powder coat instead of paint. Lower environmental impact and more durable than paint. A clear coat over decals will help protect them from damage, but whether or not you can clear coat over decals depends on the decal. Check with the supplier.