I like the idea of masking to protect the surrounding area. Also try some dry brush and stippling. Brush out most of the paint on the brush and just drag the brush with enough pressure to graze the surface for dry brush. Use a short bristle brush with a wide flat end and again brush out most of the paint and just tap the end on the surface to stipple. You can also use a natural sponge to dab some rust color or the base color showing through. use a rag coated with thinner and wad it up and lift some of the slightly cured paint. I would start with a good practice run. It will be a lot better if you work out what you want to do before you get to the real door. Oh, try dragging something like a thin broom straw through the wet paint to look like cracking. Let it dry and lift by selective sanding.
Let us see what you come up with. http://rides.webshots.com/photo/2016717540103315458ntkpbS

Neil

Neil
http://community.webshots.com/user/neilhatten