its honestly not a thing to do for your first try at body work...

because of the area you a filling you'd idealy need to remove the strip,take your edges down, weld in plates to prevent cracking, then a small filler wipe with fiberglass...but with the doors being so thin it is VERY easy to warp them and end up with a big mess. some people have just filled to whole area with a short strand fiberglass, but this WILL crack over time and will noto really end up with a good flush look anyhow.

there are also some 'super fillers' that a lot of minitruck guys use, like fuzear and other kevlar fillers...these may last a little longer but will probably still crack if done with out filling plated.

basiclly anything you wipe on is really designed to fill, not to build up...

most people consider it more of a hastle than its worth, a warped door is a pain in the ass.