Goodness knows what system works best - you'd have to undertake a lot of comparative studies using different systems and woods then sit back and wait and watch for ten years. I use Dulux Weathershield Trade exterior gloss for outdoors, and for bare wood they recommend Primer + undercoat + gloss. However, if it's untreated wood they recommend their Primer/Preservative + undercoat + gloss In fact, I used wood preservative + shellac knotting + primer + undercoat + gloss + gloss and it took bloody ages ( fascia boards, untreated pine, I didn't want to get up a ladder and remove the guttering again for a long time ). If you're painting an oily hardwod, they recommend using their aluminium primer.
For wood that I'd heat-stripped back to a kind of gungy brown original
1930's undercoat ( it wouldn't come off, just smeared out when I scraped at it ), I just went for an undercoat + gloss ( soffits, so not as exposed as the fascia boards ).My understanding is that for painting on wood, the function of the preservative is obvious, the shellac knotting seals any sap in the knots so that when the sun heats the wood it doesn't puncture the paint film by oozing sap, the primer seals the wood surface ( against what I'm not sure, I've never seen undercoat sink in when applied to bare wood ), the undercoat matches the colour of the topcoat and also provides a suitable surface for the topcoat to adhere to, and of course the gloss topcoat provides the cosmetic finish and the weatherproof seal. Two more points, Dulux claims their undercoat is elastic, and considering how wood moves, that must be a good thing. Second, they claim their gloss is microporous so the paint can breathe, and that this resists flaking. Who knows. If you use the full system it'll take you more than a week to paint any surface, but I reckon it's overkill except on exposed exterior woodwork.
My top tip is this; when you paint a surface with a sharp edge or corner, the paint always is thinnest on that edge, in fact if you try it with a white undercoat and black top coat, I reckon you can see through the topcoat on the sharp edge, and this thinness may explain why flaking often starts on an edge, so sand or plane all edges to a chamfered or rounded profile before you start,
Andy.