[Botietruck]

When I had to replace my door bottoms, while it was wide open I blasted the inside of both doors. But if you are not doing that, I would clean it very good with wire brushes, 80 grit paper or anything else you can get in there.
Now, after I blasted mine and welded in the new door bottom I used an epoxy primer and used a small paint brush and brushed the entire inside of the door. The next product I used is called Gravitex. It's the stuff body shops use to spray rocker panels and other areas to protect the surface from gravel chips. It's thinner than undercoating and it dries rock hard. It comes in a 1 liter bottle and it can be applied with a special spray gun that the supplier sells. I bought a case of the stuff ( about 6 bottles) and they gave me the gun free. A case is about $75.00. The spray gun has a long tube that will allow you to get it in all corners of the door by going through the window opening, the latch openings and the holes in the inside face of the door. It really does a great job. Just make sure you do not block the drain holes in the bottom of the door.