I have an exterior steel door which I have stained and woodgrained. The the stain keeps lifting from the door. I have done this procedure twice now with the same results. The first time I used a mimwax gel stain with a polyurethene sealer. The second time I use and Old Masters refinishing kit with a spar urethene finish coat. Is this something That is inpossible to do. I purchased the door because I wanted the woodgrain look without the maintenace of a wood door. The sales person claimed I would have no problem as this was a stainable door. I live in Iowa so there is a large tempeture change. If this is not posible what or my other options?