If it is chipped off why do you need to remove it?
If there are multiple layers of old chipping paint, be careful - some of it could be lead paint. You do NOT want to sand lead paint. In fact, I wouldn't mess around myself with lead paint at all. I'd get rid of the door - have it removed by a disposal company to be processed in accordance with hazardous waste rules. You can buy a lead paint testing kit, they aren't very expensive and are simple to use. If there isn't any lead paint, use a citrus-based non-toxic stripping product and be patient when using it. The longer it sits on, the better it penetrates. You can expect that you may have to put on several applications, but it is the safest way to get rid of the paint layers and you won't have to wear a breathing mask to avoid toxic fumes from certain types of paint strippers. Put the non-toxic stripper on, let it sit per directions, and then scrape off the gunk with a putty knife and scrape the knife off into a coffee can. Repeat until there is only a bit of residue left on the wood that you can sand away. The alternative that may be more costly but much faster (depends on the price of your stripper and how much you end up using), it may just be more economical to have your door tank-dipped to totally strip it. Check to see if there are services in your area that do this - some professional furniture refinishing businesses do this.
You should try to apply a paint remover because it may leave various grooves if you try to paint over it if it is not sanded and cleaned properly.