2) Apply knotting - this is important (if you've got big resinous knots in the timber and you want to do to do a proper job, cut out & fill, or "sweat" them at least, but knotting is OK for most knots).
3) Primer.
4) Undercoat, probably x2.
4) Gloss.
Well, that's the best way. Alternatively, chuck acrylic primer on, then acrylic "gloss". I don't like this stuff, though. The easiest thing to do is probably roller the lot with emulsion paint, or varnish them.
As Chris says, preparation then knotting then primer.
I'd differ from there, though, and say that if you don't have a particular preference for high-gloss woodwork then go with two coats of a decent trade eggshell (personally I'd always go with oil based - I really don't like painting with acrylic based stuff).
It doesn't require undercoat, and IMHO stays looking good longer & highlights definition in door panels/beading better.
Dulux is v. expensive. Leyland & Johnstones are good IME and a _lot_ cheaper. Haven't tried Crown, McPhersons or others though.
My personal choice is: Dulux trade quick drying wood primer undercoat, 1 coat (for speed) Dulux trade undercoat (for opacity), 1 coat Dulux trade eggshell (not the quick drying one), 2 coats (for durability)
The primer is water based so quick to dry, others are oil based so take an age. Probably not the quickest but this combo it is my fave for good looking durable finish. Eggshell is sort of low sheen silky and can be mixed to any colour, IIRC, the undercoat is available in white for light top coats and grey for dark colours.
Acrylic (water based) top coats are quicker but I don't rate them for stain resistance or durability, something I think is important for doors.
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