On these MB's, the whole lower part of the oustide door is covered by plastic.

There are clips at the bottom of the driver's door that hold the plastic to the metal door panel, but the clip at the rear is broken.

Another more serious problem is two small areas (about the size of a penny) of rust.
Right above the plastic, rust is growing.
It still looks white, but I know rust is inside the white coat, because the paint surface is bloated.

I thought of repairing it my self with POR-15 or RustBullet, but I have no experience in removing this plastic door trim.

Furthermore, at the top of this plastic, there is rubber strip. If the painted surface is not flat, water will leak. But I am not good at making the area flat, where I partially applied rust inhibitor and MB touch up stick.
I can use sandpaper to make it flat. However, factory clear coat (and white coat) will be sanded too, around the spot where I applied rust inhibitor.

Any advice?