My 99 430 has had the EXACT same filler door rust. I have repainted it twice. The new MLs have plastic doors (thankfully). I was able to get some color mathced rattle can paint and clearcoat over the web, and it woks pretty well. Can't get it quite as shinny, but since the door is seperate from the rest of the fender, it is hard to notice.

If you do take it off and re paint it, you will need to get the 4 plastic "rivets" that hold the inner trim piece on (the thing with tire pressures and the lock holder. Only way to take them off to paint the inside back is to break them off. You cannot just leave that part off, as if you do, the door closes too far.