polyurethane will crack and do what its doing now. Oil poly will especially when in direct exposure to sun and heat and weather.
I dislike oil when possible because it does turn the finish a light yellow color.
Water poly will not. Water poly is just as strong as oil and will allow you to apply more coats on quicker as it will dry in about 2 to 3 hours.
You do have a continous maintenance job with this door. Do you have a brass kick plate attached to the bottom? Those do help allot.
I would sand the door down and get all those fusions off and away. Then start with water poly.
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