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Paint bubbles for a few reasons. 1. water in paint or on door, 2. improper prep of the door. 3, incompatible paint to the material that the door is made of.
My thoughts first off was the door threshold stained at one time? Perhaps it contains a oil
The oils in the stain or on the item being painted will prevent proper adhesion of the paint. This oftentimes results in fish eye in the paint. We see this a lot when people fail to prep their kitchen cabinets and the grease in the kitchen prevents the paint from sticking.
Sanding the threshold is not the answer, although it help and should be considered when doing this. However, You need to clean the item real well, prior to sanding. If not the sanding will push the oils deeper into the surface making it even harder to remove.
Some plastics, but after reading your question, I think your painting wood, just will not except just any sort of paint. So you need to check the materials that the paint was designed to cover before doing anything else.