"Fixing a dinged up door" (without stripping all the paint.)
*Start by getting inside the door and hammering the ding out as best you can.
*Try and make it as smooth as possible but make sure that the ding once hammered does not become like a pimple bulging, if anything it has to be more indented then protruding.
* Sand the panel back over the entire surface to be painted, and over the dinged area and a area around this also ruffen up with a course sand paper or a pannel beaters file.
*Apply body filler to the damaged are and smooth it as best you can to match the surrounding area of the panel.
*Once dry use the file to sand smooth with the panel following the lines of the car.
* Repeat the process again, but this time a file should not be required, instead use a sanding machine or a sanding block to make sure it is all looking the same as the door. you may need to do it a third time to fix all imperfections.
* Make sure it all feels smooth and blends into the panel of the door smoothly with no join lines.
* Use a Etch primer preferably and spray a smooth coating over the whole door after it is taped.
* Sand it smooth once dry it should all look really smooth with no imperfections.
* At this point you can spray a spray putty lightly over areas if there are still imperfections and sand smooth.
* By now it should look and feel amazingly smooth.
*time for painting.
Spray 2 to 3 coates of colour dont spray to close as it will drip and not top heavy and it will drip. light coates are better. The first coat acts as a bond for the second and third, so just lightly on the first. ( patchy is ok)
* Do the second and third.
Non metalic or non pearl paints after this time the painting is complete and requires to be left to dry.
The metalic or pearl paints require a Clear coat to be painted after the colour.
so 3-5 coates can be painted.
Leave a good week to dry and cure.
* Buffing is your next step, use a wet and dry sandpaper start with a course grade to a fine grade and sand slightly to remove noggings of paint and paint feels smooth.
* Use a T-cut or colour restore product and lots of elbo grease and do all painted area, buffing after.
* The paint should now have a high gloss.
I hope this helps, I am no pro or panel beater! But this is how I would do it and done so with excellent results many times.