Every time you repaint a surface, a little more thickness is added to the existing layers. This build up of successive layers of paint can eventually lead to problems, including clogging of detailing and the moving parts of doors and windows start to bind and catch against their frames. Anyone with an old house that has double hung sash windows will know exactly what I am talking about. When this happens, it is time to strip the paint back to bare wood and start again.
In the past using a blow torch was the most popular method to strip paint, however these days electric heat guns are commonly used. Alternatively, a chemical paint stripper can be used. Most Paint strippers contain dimethylene chloride or caustic soda. You must wear the correct safety gear when stripping paint. Heavy duty Industrial gloves, Safety Glasses, an Apron or disposable coveralls and steel cap boots.
You should have access to running water, in case any of the chemical gets on your skin or in your eyes. Wash with copious amounts of water. If you get any in your eyes go to a doctor asap.
Another thing to be aware of when stripping paint from old buildings, is the possibility that there will be lead based paint in a prior coating. Where possible the area to be stripped should be encapsulated, so as to catch the old paint for easy collection and disposal when you finish.