how to strip the paint from a bike yourself? How can I strip the paint from a bike frame?
I tried using a sander and it left really annoying scratch marks on the frame.

Is it possible to use a chemical paint stripper to completely remove the paint from the frame? 

I do not plan to repaint it after stripping the paint.
John Allen has a page on refinishing a bicycle on Sheldon Brown's website. You need some covering on the frame to protect it, though. For aluminum, it is recommended to use chemicals and not sandpaper (due to particle embedding), but for steel, sandblasting is fine. The main issue with chemicals is fumes. 

There are also several related threads on bicycles.SE such as this one and this one.