I hope that the door has a proper threshold on its bottom, because a threshold of oak is the only thing that will make the door weatherproof. If not, have one installed. First get rid of the plaster, which is not the thing to use outdoors. It depends on how thick the gap is. If a threshold, sloped to allow water runoff, fits in the gap, that is good. If there is a little more to the gap, put in a pressure-treated board to make up the difference. If there is still a gap, you can build a berm of concrete or bricks to close the gap.
I live in a three-decker which has back porches off the second- and third-floor units. The third floor has a tongue-and-groove deck (we’re told it was replaced 10 years ago) that leaks badly. Of course, both porches get wet, but the rain is clearly leaking through the boards. Also, all that damp is peeling the paint on the bottom of the third-floor deck. A contractor told us the problem is that the third-floor porch is laid flat and needs to be rebuilt on a slight incline. Do you think that will fix the problem?
Yes, if the third floor deck was sloped a little, it would make a big difference. I would have to guess the third-floor deck is covered by a roof, as in all triple-deckers I know. All three decks should be primed and painted with an exterior primer and exterior deck paint.
We had our floors sanded and polyurethaned six years ago. Our foldable ironing board gradually scratched a small area. Can this section be locally sanded to remove the scratches and polyurethaned only on the affected area?
Yes, sanding the scratched area lightly and applying two or three coats of water-based polyurethane varnish on the scratched area can work. So can coating the scratches with a stain just a little darker than the scratches.