Who ever comes up with this Backward saw blade to do cutting, This is burning as you are creating friction to do the job and a bad job at that... Whoever it is who "They" say puts the blade on backward, I wish "they" would shut up and use common sense. Get the correct blade. If it were me, I would get a metal cutting blade for my 4" side grinder and saw along, Or Metal cutting blade for my Jig saw and cut along a line using a straight edge to control the cut. Or get a metal cutting blade for my skil saw, using a straight edge to guide my cut. Or get a metal cutting blade for the Sawsall... There are a load of safer methods than the backward blade.
Think about it, If it is a carbide blade, you will rip the teeth off, If not a carbide you will burn the teeth off. So where are you? hot burnt ruined mess of a blade and only a small burnt mark on the door.
Wear safety Glasses and shield and long sleeves and gloves as there will be sparks, have a fire extinguisher on hand. After the cut grind/file the edges and insert a wood block filler to fill the void as you most likely cut off the core inside the door.