How can I get rid of a stain in my ceiling that was caused by a leak in my roof?

When I prime my outside wood, how long do I have to wait before painting? Can I paint right away?

I applied modern oil base paint over older lead oil base paint and I am finding areas where the new oil base paint is peeling. What is causing this?

I am buying a home that has brown paneling interior doors. I can not afford to replace them with new doors. So I am thinking of painting them white. How do I go about doing this? What type of paint do I use?