A vinyl coated steel door is a good choice for an exterior door. These doors are lightweight and durable, and the vinyl coating protects against moisture damage. Unfortunately, a vinyl coating does not accept paint as easily as other more porous surfaces. If you want to change the colour of your vinyl coated steel door, you need to properly prep the door to ensure the paint will provide lasting coverage.
Remove the door from its hinges if possible. If the door is difficult to remove, or if you don't have help for lifting and carrying the door, make sure you protect the nearby trim with masking tape. Additionally, use very thin coats of paint to prevent dripping.
Place the door on top of a protective tarpaulin.
Sand the door with 320-grit sandpaper. Most vinyl surfaces can tolerate slightly rougher 220-grit sandpaper, but you do not want to risk even moderate exposure of the steel, as steel surfaces require different primer when compared to vinyl.
Wipe down the surface with a damp rag to remove the sandpaper dust.
Dry the door with a towel, or wait for it to air dry.
Apply a bonding primer to the door using a paint roller. Bonding primers are better for slick surfaces than general purpose latex varieties.
Use a foam brush to apply primer to the edges if you were unable to remove the door. A detached door can be completely painted with a roller, but a foam brush works well in tight spaces if you have nearby trim or exterior siding to contend with.
Let the primer dry. Flip the door over and prime the other side.
Use a clean roller and foam brush to apply a latex acrylic paint onto the vinyl coated steel door. Avoid dark colours for the exterior side of the door, as these absorb more sunlight and can damage the underlying vinyl.
Flip over the door after the first side dries, and paint the backside of the door.
Wait for the paint to dry and apply additional coats as necessary.
Reattach the door and remove any masking tape after the final coat fully dries.