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Before painting an old door, it's important to determine if the original paint is oil-based or latex; latex paint will not adhere to oil-based paint unless certain preparatory work is done. Soak a clean rag in denatured alcohol and rub the surface of the door. If the paint starts to soften, it is latex. If it doesn't, the paint is oil-based. This door is oil-based.
To apply latex over oil-based paint, clean the door thoroughly. Use 150-grit sandpaper to sand all the rough spots (Image 1). Follow with 220-grit sandpaper to sand the door nice and smooth (Image 2). Clean all the dust off door.