The old paint on my garage door is powdery but not peeling off.
> What is the best thing to do before over painting it ?
> Presumably I need to seal it with some sort of primer before over
> painting it .... what is best ?
I don't think it makes any difference whether metal or wood in this case. You could of course strip it right down but from what you say it doesn't seem necessary. What I did with my own garage door is first make sure the old paint is thoroughly clean - using a softish broom and Mangers in a large bucket, then rinse liberally with water. (I find a broom ideal because it gets into any grooves as well as being practical on such a large surface.) Obviously allow to dry thoroughly then sand all over the door very well, making sure there's absolutely no loose paint. After that use the undercoat and topcoat. I stand to be corrected but I think you only need primer if the bare wood or metal is visible.