Mapei primer is a low-voc water-based primer. It isn't particularly strong, so it shouldn't be too hard to remove. Pick up a heavy-duty paint stripping solvent and use a disposable brush to cover the primer in your stripping agent. Read the label on the solvent to see how long it needs to soak and then use a putty knife to scrape the stripper off. The primer should peel right off of the door.