The material and/or methods you use will largely depend upon the type of paint you're looking to remove. Given the photo I'm assuming it is latex/acrylic over old oil base paint. Heat strippers work well on old oils, you just have to be very careful about holding them on too long and actually burning or charring the wood below -- not a good idea if you plan on staining this as part of the refinish. For acrylics and latex go with the chemical stripper mentioned above. I have used this material with very good results. However, because this is a very strong alkali you should follow up with a mild acid wash -- 2-3 oz. white vinegar in 1 gal. water. This wash will raise the grain some but that's really to your advantage since you will want to do some finish sanding before you prime or stain the door. Having good, sharp paint scrapers will make the job that much easier, and at least one should be carbide. BTW - when using the chemical stripper work in a WELL ventilated area, wear your respirator, and invest in some good rubber hazmat gloves. This stuff is nasty if it gets on your skin especially if exposed to direct sun.