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I used outdoor paint for wood on a metal door that goes from the garage into the back yard. Is there a finish I can put on it (polyurethane?) that will keep the paint from coming off, or do I have to refinish the whole door.
As long as you properly prepped the door and the paint was exterior grade, it should be fine.
If you prepped the door with a metal grade primer like rustoleum red or an automotive grey primer, or my favorite, zinc chromate, then the house paint will be fine. Going over the top of the house paint with polyurethane will not help the house paint adhere to the door if the proper prep wasn't done.
Some of the new house paints are top coat and primer in one and I see new steel doors painted with this so maybe it will be enough, time will tell. What a poly topcoat will do is preserve the finish longer and most poly topcoats have a UV inhibitor in them so the color won't fade as fast.
You can increase the durability just as easy with another coat of the house paint and if the door is the same color as the house and you coat it with poly, the rest of the house will fade and the door will not, eventually resulting in a mismatch.