When we purchased our place in 1976 we began to restore old surfaces. The rear doors were caked with many many layers of paint, probably some with lead in it.
We tried and tried to clean them up to no avail. Finally I bought a heat gun, which is good for vertical surfaces outside if the paint will cooperate. To get it to cooperate and hold together at removal I found that coating with Kilz and letting it dry made a kind of glue or frame for the paint underneath and after using Kilz to cover over the old paint my heat gun could easily loosen an area that I could simply peel away with a good scraper.
It took many hours but now my back doors are pristine and ready for a good paint or stain job which will take them back to original.