I want to restain my faded wood garage door. The surface is smooth and doesn't appear to need sanding. After cleaning it with a solution of TSP and water and rinsing and drying the door,I notice that when I rub the surface, it leaves a mark as though there is a fine film on the door. I have also tried to clean with paint thinner, but I can still leave rubbing marks afterward.
Question 1. What causes this?
Question 2. Can I safely go ahead and restain the door or do I need a sealer? If so, what kind or brand of sealer would be best?