I think I read somewhere that they don’t use regular gloss paint on number 10. Loads of undercoats (as you recommend), then several coats of some kind of buffable black lacquer, with a clear polyurethane top coat. Probably sprayed, as you say. As expensive as the wallpaper no doubt.
If you're ever painting a front door gloss black people passing can't resist a 'No. 10' comment. :)
It's surprising how much hotter black doors get from the heat of the sun. Most of the resin/knot problems I get asked about are on black or very dark colour doors.
I think I read somewhere that they don’t use regular gloss paint on number 10. Loads of undercoats (as you recommend), then several coats of some kind of buffable black lacquer, with a clear polyurethane top coat. Probably sprayed, as you say. As expensive as the wallpaper no doubt.