I stained my front door with several coats of stain. Once it dried applied polyurethane to clean dry surface and a white color appeared on most of the door. How do I remove the color and refinish the door?
The white "color" could be caused by several coats of stain. The many coats of stain takes several days to fully dry and when the polyurethane was applied it trapped the solvent still in the wood. Another cause is humid conditions during the application. This discoloration is called blushing.
Try using fine, #0000, steel wool and linseed oil. Rub the affected areas in the direction of the wood grain until the white spots go away. This works with lacquer, most of the time, but I don't know about polyurethane. If this works you will need to polish the door to return the sheen to the rubbed areas.
If the spots can't be rubbed out then you will have to refinish, start over.