Prepare
Take off the hardware, sand, undercoat
Unscrew the knocker, letter plate, handle and number rather than masking them: the result is in another league. Wash the door with degreaser, rinse. Sand flaking areas back to sound wood with 120 grit, then the whole door with 180 to dull it; fill splits and holes with exterior wood filler and sand again.
On bare wood, filled patches and a new door, apply our Multi-Surface Primer in the tint matched to your colour: under Market Green or Blueprint, the dark-grey version saves two topcoats. A steel door is sanded very lightly and takes the same primer.