How many coats of exterior gloss?

Yes, you will be fine with one top coat but 2 coats would not do any harm. You mention Dulux...... use the Dulux Weathershield paint as this will give you up to eight years protection, important to follow the instructions on the container.

 

can I get away with one top coat?
tyddewi

You make it sound as if it would be wrong to only give it 1 coat of gloss, when in fact, it's the right thing to do. Dulux Weathershield and 2 coats undercoat would be your better option but as you're going to do it black it will soon suffer with the sun. If you've any active knots...black gloss will find them.
Regular maintenance is better, than an all out "gotta do the job properly" every once in a while.

White is the new black.

 

Just to echo what been said here, Dulux Weathershield is the dogs.

Do the job properly, all the carp off, knotting if necessary, prime 2 u/c and gloss - can't be beaten.

I did the front of a previous house like this some years ago, after 8 years was still as good as new, it was regularly washed down. went back 5 years later, the house had been repainted - poorly - and the paint I applied was still visible underneath and it looked to be in good condition.

 

2 coats. always the best approach.

 

Johnstones Stormshield flexi gloss - guaranteed 8 years
Sandtex flexigloss - BBA vertified 10 years durability.

£10 cheaper per can.

Johnstones is owned by a £15 billion dollar multi-national who just make paint under a wide range of names. (I am not sure though if weathershield is more gel like and so easier to apply thickly)

 

Weathershield gloss is a liquid paint, not gel like.

 

I had a special gel version, known as a cold pot.

I do not know exactly why I and two other guys, both with 30 years or so experience (one even taught decorating) have had problems with adhesion despite proper prep with the matching undercoat (and I think it goes for all flexi gloss), but it might be due to damp air and cold surfaces, or air bubbles, or not quite set enough undercoat before topcoat.