How to restore hollow metal door?
On bare metal, as long as the dents are very minor, I would skim coat the surface with bondo and use a large sanding block to sand the surface flat.  Be sure to do a fine grit sanding after primer and it should come out looking good.  If there are dents that will require the bondo to me more than 3/16" at deep any point, you will need to correct those (pull them out) first.