I have recently painted a teak door and agree with what NHG suggested. Firstly, I sanded the door (the hardest part of the job!!), then wiped with White Spirits, then applied the aluminium wood primer followed by a coat of undercoat and then the gloss (green).
I painted the inside of the door white, so internally I used a white primer, not sure if this was correct, but it seemed to work ok.